Naps, cognition and performance

Abstract

Daytime napping is a frequent habit of many individuals, whether healthy or not, and may occur in a wide variety of contexts. There are several reasons for napping in the human adult, including prophylactic strategies or recuperative need, respectively before or after sleep loss, or even pure appetitive drive. Thus, it is of great theoretical and clinical interest to assess the impact of naps on individuals' performance, especially on cognitive functioning. As the outgrowth of a symposium held by the authors at the 5th Congress of the World Federation of Sleep Research and Sleep Medicine Societies in Cairns, Australia, September 2007, this review will specifically explore: a) the newly developed experimental daytime split-sleep schedules and their effects on recovery, compared with those deriving from a single consolidated sleep episode of equal duration; b) whether naps may be beneficial to wakefulness performance in the working context, through accurate review of "on field" studies; c) the impact of naps on cognition, in light of the very recent advances in the study of naps and memory processes; d) the main features of napping behavior in older individuals and its impact on their health and general functioning, since it is widely recognized that napping may change as a result of the aging process.

Full text

CLINICAL REVIEW
Naps, cognition and performance
Gianluca Ficca
a
,
*
, John Axelsson
b
, Daniel J. Mollicone
c
, Vincenzo Muto
a
, Michael V. Vitiello
d
a
Department of Psychology, Second University of Naples, 81100 Caserta, Italy
b
Karolinska Institutet, Department of Clinical Neuroscience, Section for Psychology & Osher Center for Integrative Medicine, Stockholm, Sweden
c
Division of Sleep and Chronobiology, Department of Psychiatry, University of Pennsylvania School of Medicine, Philadelphia, PA, USA
d
Departments of Psychiatry & Behavioral Sciences, Medicine (Gerontology & Geriatrics), and Biobehavioral Nursing & Health Systems, University of Washington, Seattle, WA, USA
article info
Article history:
Received 7 April 2009
Received in revised form
16 September 2009
Accepted 17 September 2009
Available online 2 December 2009
Keywords:
Aging
Cognitive functions
Memory
Naps
Performance
Shift work
Sleepiness
Split-sleep
summary
Daytime napping is a frequent habit of many individuals, whether healthy or not, and may occur in
a wide variety of contexts. There are several reasons for napping in the human adult, including
prophylactic strategies or recuperative need, respectively before or after sleep loss, or even pure appe-
titive drive. Thus, it is of great theoretical and clinical interest to assess the impact of naps on individuals’
performance, especially on cognitive functioning. As the outgrowth of a symposium held by the authors
at the 5th Congress of the World Federation of Sleep Research and Sleep Medicine Societies in Cairns,
Australia, September 2007, this review will specifically explore: a) the newly developed experimental
daytime split-sleep schedules and their effects on recovery, compared with those deriving from a single
consolidated sleep episode of equal duration; b) whether naps may be beneficial to wakefulness
performance in the working context, through accurate review of ‘‘on field’’ studies; c) the impact of naps
on cognition, in light of the very recent advances in the study of naps and memory processes; d) the main
features of napping behavior in older individuals and its impact on their health and general functioning,
since it is widely recognized that napping may change as a result of the aging process.
Ó2009 Elsevier Ltd. All rights reserved.
Introduction
Daytime napping is a frequent habit in many individuals,
whether healthy or not, and may occur in a wide variety of contexts.
Seminal research on naps, extensively reviewed in a volume by
Stampi,
1
has tried to address the regulatory mechanisms sustaining
polyphasic sleep structure, as well as to identify the determinants of
napping, on both the psychobiological and the psychosocial level.
These studies have focused on several different reasons for napping
including: recuperative need, due to prolonged wakefulness or
increased sleep pressure; prophylactic strategies, aimed to coun-
teract an expected sleep deprivation and to maintainperformance in
particular contexts such as shift work or sustained operations; and
pure appetitive drive, linked to sociocultural and individual char-
acteristics. Thus the causes of napping are likely to be multifaceted,
a picture made even more complex by the continuous interactionsof
each of these factors with the others. However, the question of what
impact napping might have on the general functioning of individ-
uals, with special regard to wakefulness performance and memory
processes, has so far received little attention.
This review seeks to raise interest in these theoretically and clin-
ically fundamental aspects of napping. It is the outgrowth of
a symposium, ‘‘The Effect of Naps on Health and Cognition’’, held by
the authors at the 5th Congress of the World Federation of Sleep
Researchand Sleep Medicine Societies in Cairns, Australia, September
2007, specifically conceived to pay thorough attention to the rela-
tionships between daytime napping and cognitive processes, in light
of the very recent advances in the study of naps, memory and
performance. Here we will examine the interrelationships of napping,
cognition and performance in four specific contexts.
First, the hypothesis that a split-sleep schedule provides more
recovery than a single consolidated sleep period of the same total
duration is examined. We will start with this particular approach
since it represents the most recent experimental contribution, after
the classical 80s studies on daytime sleep regulatory mechanisms,
1
to set the basis for the understanding of nap effects in ‘‘real life’’
situations. Second, the advantages and disadvantages of napping in
the work environment are examined. Third, the purported benefits
of napping for the learning of new material, either declarative (e.g.,
lists of words) or procedural (e.g., perceptual or motor tasks) are
explored, including a discussion of why daytime naps, due to their
peculiar sleep infrastructure, might represent a useful model of
memory consolidation mechanisms during sleep. Finally, the
prevalence of regular napping in the elderlyand its association with
*Corresponding author. Tel.: þ39 823 274790; fax: þ39 823 323000.
E-mail address: gianluca.ficca@unina2.it (G. Ficca).
Contents lists available at ScienceDirect
Sleep Medicine Reviews
journal homepage: www.elsevier.com/locate/smrv
1087-0792/$ – see front matter Ó2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.smrv.2009.09.005
Sleep Medicine Reviews 14 (2010) 249–258

sleep complaints, excessive daytime sleepiness, and mental and
physical health problems are examined, with a focus on whether
regular napping among older adults, particularly those in good
health, may be beneficial to daytime wakefulness or detrimental to
night-time sleep propensity.
Effects of split-sleep schedules on sleep stage architecture and
neurobehavioral performance
Can split-sleep enhance recuperative benefits to performance?
The idea of splitting a sleep period up into two parts of the
same total duration to provide enhanced recuperative benefits
was first posed in 1897 and came from observations about the
exponential time-course of sleep depth from measured auditory
arousal threshold.
2
The idea was that exponential functions are
steepest in the first half of the function (i.e., provide more
recovery per unit time invested in sleep), thus breaking sleep into
two parts would exploit this property advantageously yielding
more recovery than a single sleep period of the same total dura-
tion. In recent decades, others have reported that alertness and
performance are restored as a saturating exponential function of
total sleep,
3,4
however only a small number of studies have
directly compared split-sleep and monophasic sleep to determine
if in fact there are neurobehavioral benefits of splitting sleep into
parts. We begin with a review of the factors related to split-sleep
schedules that impact sleep architecture and neurobehavioral
performance including circadian timing of sleep, duration of prior
wakefulness, and cumulative sleep loss.
Factors related to split-sleep that affect sleep architecture
Circadian timing of sleep has been extensively examined in
experiments involving long-term temporal isolation, multi-cyclic
sleep wake schedules, and forced desynchrony. These experiments
showed that circadian phase as well as the duration of prior
wakefulness affects sleep propensity (i.e., sleep onset latencies and
sleep duration) and sleep architecture by primarily impacting rapid
eye movement (REM) sleep. In fact it has been shown that there are
certain circadian ‘‘forbidden zones’’ where nap sleep initiation
would be improbable at normal homeostatic levels.
5,6
For an
extensive review of these experiments see the review by Dijk and
Czeisler.
7
Sleep loss resulting from total sleep deprivation and
chronic partial sleep deprivation has also been shown to affect
sleep propensity and sleep architecture.
8,9
Experiments about sleep
loss effects on sleep architecture are reviewed by Dinges et al.
10
Factors related to split-sleep that affect neurobehavioral
performance
In order to fully characterize the impact of napping on neu-
robehavioral performance associated with split-sleep perfor-
mance, factors such as mood, cognitive performance, and motor
function must be examined across a range of homeostatic levels
resulting from both acute total sleep deprivation and chronic
partial sleep deprivation. It is known from controlled laboratory
experiments that sleep loss resulting from acute total sleep
deprivation negatively affects mood, cognitive performance, and
motor function due to an increasing sleep propensity and desta-
bilization of the wake state.
11
Sleep loss resulting from chronic
partial sleep deprivation progressively impacts these same factors
in a dose-response relationship with TIB across days of sleep
restriction.
9,12
The duration of prior wakefulness at the time of
testing has been shown to impact performance and to interact
with circadian phase.
13
The important theoretical question is
whether split-sleep schedules mitigate neurobehavioral deficits
resulting from homeostatic pressure associated with sleep
restriction when compared to consolidated sleep. Sleep inertia
must also be accounted for in the interpretation of experiments
designed to measure the impact on neurobehavioral performance
of split-sleep schedules. Sleep inertia impacts neurobehavioral
performance for 2 or more hours after waking
14
and is most
pronounced at adverse circadian phases in the middle of the
habitual night.
15
It has been shown to increase in magnitude and
duration with total sleep deprivation and can be mitigated by
caffeine.
16
Finally, experimental evidence has demonstrated that
there are large, stable, trait-like differences among individuals in
the amount of daily sleep required to maintain stable levels of
performance.
17
In fact, individual differences in the response to
sleep loss should be considered when studying any of the above
mentioned factors related to split-sleep schedules.
18
Experiments that directly compare consolidated and split-sleep
schedules
We now review experiments that specifically compared
consolidated sleep with split sleep. In all of the experiments
described in this section cognitive performance was assessed using
objective measures such as a digit symbol substitution task, mental
arithmetic tasks, or simple and choice reaction times tasks. Mood
and sleepiness were assessed with subjective scales. The first
experiment considered was a within-subjects design by Nicholson
et al. that compared 8 h of continuous nocturnal sleep to a split-
sleep schedule that was comprised of two 4 h sleep periods
bisected by 10 h of nocturnal wakefulness.
19
No differences in
performance were detected in subsequent daytime performance
between the consolidated and split-sleep schedules. Other exper-
iments examining split-sleep compared to consolidated sleep of the
same total duration found increases in subsequent daytime
performance; however these effects were attributed to experi-
mental confounds related to differences in duration of prior
wakefulness at the time of testing.
20,21
A recent controlled labora-
tory experiment by Dinges’ group examined performance impair-
ments associated with a range of split-sleep schedules that were
comprised of restricted nocturnal sleep augmented with a daytime
nap or no nap. In order to minimize confounds related to circadian
phase and duration of prior wakefulness, data were analyzed based
on daily-average-performance to identify the functional relation-
ship between performance and nocturnal anchor sleep and diurnal
nap sleep. Tests that occurred immediately after awakening from
a nocturnal sleep or diurnal nap were not included in the daily
performance averages to avoid measuring sleep inertia effects. The
overall finding was that performance was a function of total daily
time in bed independent of whether sleep was consolidated or split
into two parts.
22
In terms of total sleep time and neurobehavioral
performance, it did not substantively matter whether sleep was
consolidated or scheduled in two parts.
22,23
Less is known,
however, about any health impacts of any types of split-sleep
regimen, even if this strategy of sleep would be favorable for
maintaining performance.
Effects of naps on vigilance/performance/subjective well-
being as assessed from ‘‘on field studies’’
With respect to napping in working life, most naps occur in
association with night work. Although the majority of naps seem to
be compensatory in nature (i.e., a result of sleepiness/sleep pres-
sure), the large number of naps occurring before the first night shift
demonstrate that many workers also take prophylactic naps to
reduce night-shift sleepiness.
24,25
Some studies report that up to
G. Ficca et al. / Sleep Medicine Reviews 14 (2010) 249–258250

65% of the workers take a nap before the first night shift, and this
number may increase up to 91% if preceded by a morning shift the
same day.
26,27
After the first night shift, the numbers of nap takers
are reduced, to about 10–40%,
25,27
mainly because the main sleep
bout now occurs during the day. Napping on the night shifts is
strongly affected by occupation, culture and whether the naps are
sanctioned or not.
24
While the literature is largely limited to
voluntary naps, night shifts are also affected by involuntary naps. It
has been estimated that the risk for unintentional sleep increases
by about 60% during night shifts.
28
In fact, in a larger diary study,
with self-reports of 230 train drivers and air traffic controllers, up
to 50% reported dozing off during the night shift.
27
Some of the
scarce objective data on involuntary naps (i.e., using ambulatory
electroencephalogram) revealed that 20% of the workers fell asleep
during the night shift, whereas none fell asleep during morning or
afternoon shifts.
29
Similar findings have also been shown amongst
pilots.
30
An important finding from the former study was that many
sleep episodes were unreported by the workers themselves, sug-
gesting that falling asleep during a night shift is more common than
in self-reports.
The main reasons for reduced alertness and performance during
the night are the circadian system, promoting sleepiness and long
waking hours. Waking hours are particularly long the first night
shift, often more than 20 h. Several strategies, such as napping,
caffeine, modafinil and bright-light exposure, have been proposed
as effective countermeasures against sleepiness and performance
decrements. However, rather few studies have actually been carried
out on how effectively these countermeasures work in real life
situations. As of today, the most evidence exists for napping, and it
has been concluded that naps can alleviate shift work problems in
a reliable and inexpensive manner.
31
Naps and caffeine are the
most common countermeasures and a recent intervention study
has assessed their combined effect.
32
Below we review the studies
carried out in operational settings, particularly regarding inter-
ventions. A final section will provide some guidelines on how to
make naps more effective.
Implementation of naps to improve late night alertness
Multiple laboratory studies have demonstrated that naps from
30 min to 4 h improve alertness and performance in laboratory
conditions of simulated night-work, e.g.,
32,33
However, in most
cases, the research samples included young volunteers rather than
experienced shift/night workers and the performance tests were
most often short performance tests such as reaction time tasks
rather than work related output or safety. In an experimental study
including oil refinery shift-workers, Sallinen and co-workers
showed that naps of either 30- or 50-min taken at 01:00 h or
04:00 h improved performance at the end of the night shift.
34
The
fact that daytime sleep was somewhat impaired by the 50-min
naps suggests that longer naps may also have positive effects on
the drive home after the night shift. Naps taken before the night
shift may also improve nighttime performance and alertness.
Clearest evidence arise from studies using rather long naps,
32,35
2–3 h, which seem obvious if the aim is to have effects many hours
later.
Effects of naturally occurring naps
In contrast to the positive findings found in laboratory studies,
naturally occurring naps have been found to relate to greater
sleepiness and poorer performance in shift workers.
25
This finding
supports the idea that most naps are compensatory in nature rather
than prophylactic On the other hand, in a study of 250 pilots, less
fatigue was reported amongst pilots stating that they took cockpit
naps or routinely took naps prior to overnight flights.
36
A recent
study of 1195 police drivers found that napping was related to
fewer car accidents.
26
Notably, almost 90% of the subjects took
a nap before the night shift in this extremely rapidly rotating shift
system. The unusually high degree of nap takers should be seen in
light of the fact that the night shift was preceded by several short
recovery periods and a morning shift the same day. Rather than
suggesting that prophylactic naps are effective against accidents,
these results highlight the serious consequences (substantial
increase of accident risk) for those 10% not managing a nap
between the morning and the night shifts if these occur the same
day. With the exception of this rather extreme working schedule,
there is limited knowledge as to whether naturally occurring naps
are really effective.
Implementation of naps in operational shift-work settings
Only a few studies have investigated the effects of nap inter-
ventions in operational shift-work settings. In 24 aircraft mainte-
nance workers, Purnell and co-workers found that a short 20-min
nap taken around 03:00 h had a positive effect on vigilance
performance at 07:00 h and on alertness when driving home.
37
A
study confirming possible long-term effects was carried out by
Bonnefond and co-workers. This study followed 12 industrial
workers having a one-hour nap possibility between 23:30 h and
03:30 h for a whole year.
38
The majority of the workers reported
reduced fatigue and improved general satisfaction. Interestingly,
the effect of the nap improved progressively over the year. These
long-term effects are rather unexpected and need to be confirmed,
particularly as no control group was included.
Napping interventions have perhaps been most successful in
extended working shifts. In a study with 20 nurses working 16 h
shifts, Takahashi and colleagues
39
found that a 2 h nap opportunity
improved alertness and reduced heart rate. In another study of 12-
hour shifts, it was shown that a 40-min nap opportunity at 03:00 h
had positive effects on sleepiness and driving simulator perfor-
mance at 07:30 h, indicating positive effects on alertness on the
drive home from work. As many as 90% out of the 49 physicians and
nurses actually fell asleep during the time interval allotted for the
nap.
39,40
Two studies have also introduced nap strategies in
medical interns working 30-hour shifts. Arora and colleagues
showed that the introduction of a nap-schedule with on-call
coverage resulted in more sleep and less fatigue.
41
In the other
study on medical interns,
42
advice to take naps before the night
shift was part of a larger (successful) intervention including the
reduction of maximum working hours to 16 rather than 30. Also
short 30-min cockpit naps may improve performance.
43
A limitation with many of the presented studies is the lack of
work output and safety measures. However, more recent studies
have included ‘‘real accidents’’,
26
driving simulator performance
after work
40
and ‘‘work logs’’.
42
Short tests are not similar to most
work tasks, but can still indicate where severe sleepiness and
improvements can be expected.
In real life situations, workers are free to combine several
countermeasures to improve their on-shift alertness. However, few
studies have investigated the combined effects of more than one
countermeasure in a systematic manner. In a recent study by
Schweitzer and colleagues
32
it was shown that an individualised
nap and caffeine intervention can be successfully introduced.
Workers at different work sites were instructed to take a 2-hour
nap at home before the first two night shifts and combine this with
a 300 mg of caffeine at the beginning of the shift. The effects
included less sleepiness and better performance at the end of the
shift. Still, further studies are required if we intend to understand
the combinatory effects of several countermeasures to maximize
alertness at crucial time points. For example, is caffeine an effective
way to reduce sleep inertia after naps on the shift?
G. Ficca et al. / Sleep Medicine Reviews 14 (2010) 249–258 251

These intervention studies have reported a series of problems
with the implementation of naps. Some workers are obviously non-
nappers.
37
Earlier studies have proposed that almost 50% are non-
nappers,
38
but the fact that up to 90% nap during an extended night
shift or between a morning and a night shift suggest that napping is
possible for the majority. On the other hand, intervention studies
may suffer from selection problems in that non-nappers tend to
abstain from participation; it is unclear how inclusion/exclusion of
participants in most of the studies have been carried out. Similar
selection biases may also be at play in night workers and it seems
reasonable that the capability to nap makes it possible to manage
strenuous working hours. Also sleep conditions are important and
noise has been reported as a major reason for disturbed nap sleep.
37
Sleep inertia is another concern as it severely interferes with
working capacity and safety in the period after awakening.
15,44
In
a study on nurses with 16-hour shifts it was shown that sleep
inertia may last as long as 2 h.
39
In addition, the beneficial effects of
brief naps may be less effective in severely sleep deprived
subjects.
45
Another problem that affects napping in real life situa-
tions is the demand on the worker. Medical interns stated that the
main reason for not taking full advantage of their nap opportunities
were concerns for their own patients.
41
One of the major concerns
deriving from laboratory findings has been disturbed day sleep
after the night shift. Although this topic was not addressed in all
studies, day sleep impairment was not found in those subjects
taking relatively short, 20–30 min, naps.
34,37
Nevertheless, a slight
negative effect on subsequent day sleep should be seen as a good
sign as this also implies improved alertness on the drive home from
the night shift.
Implementation of naps to improve alertness in the afternoon
A number of laboratory studies have shown that short day-time
naps (3–30 min) may improve alertness and performance for up to
2–3 h.
46–48
Surprisingly, Brooks and Lack found that 10-min naps
were more effective than slightly longer ones.
48
The improvements
were immediate, hence avoiding sleep inertia, and lasted for the
remainder of the 2.5 h during which the subjects were tested.
Although most studies on brief naps are limited to subjects with
restricted sleep, typically 4–6 h of night sleep, there is some
support for brief naps to be beneficial also after normal sleep.
Takahashi and colleagues found that subjects allowed 7 h of sleep
benefited from a 15- or 45-min nap at 12:30 h – alertness was
improved more than 3 h later.
49
On the other hand, a small-scale
intervention study in eight Japanese industry workers showed that
a brief, 15-min, nap at 12:30 h failed to improve performance
measured at 15:00 h in workers with no prior sleep deprivation.
50
Perhaps the results would have been different if more than eight
participants had been included. Nevertheless, the authors reported
that the short naps improved post-nap alertness at the end of the
working week, and half of the workers continued napping the
following work week when napping was no longer obligatory.
General guidelines
In conclusion, it appears that napping should be always imple-
mented when severe sleepiness is most likely to occur. For night
workers, the worst periods coincide with the circadian trough,
around 3–6 in the morning, and on the way home from the night
shift. For the day worker, naps should be taken to avoid the after-
noon dip. In most cases, the afternoon dip is rather limited and can
often be relieved by caffeine. However, daytime napping should be
seen as a legitimate alternative for those suffering from hyper-
somnia, disturbed sleep or non-coffee drinkers.
31
Intervention studies clearly demonstrate that napping can
improve night shift alertness and performance. Shorter naps seem
more successful than longer naps, although there is clearly a limited
literature about the latter in operational settings. Naps before the
night shift should preferably be as long as possible, when avoidance
of sleep inertia is less important, but avoided during work time or
other periods when high performance is required. To avoid sleep
inertia, naps should not be too long (max 20–30 min), not occur at
the bottom of the circadian phase, i.e., early morning hours, nor
taken after long waking times.
15,44
An alternative to long naps may
be several short naps, but this possibility requires further
exploration.
Napping seems a very important countermeasure during
extended work shifts and in operational settings,
51
particularly in
those contexts where safety is an important issue. Furthermore,
although most studies have been conducted in young and sleep
deprived subjects, the practical advice is that short daytime naps
are effective for those with moderately disturbed sleep and
possibly for normal sleepers. If the aim is to alleviate afternoon
sleepiness, the timing of the nap should not be too early. The
alerting effect can be further enhanced if the nap is combined with
caffeine.
52
Laboratory studies suggest that caffeine at the beginning of the
shift may also help in reducing sleep inertia from nighttime
napping.
14
However, further studies are needed to verify how the
use of naps and caffeine can be best combined in operational shift-
work settings. Lastly, napping recommendations should be an
integral part of all real life fatigue management.
The effect of naps on memory processes
Daytime naps as a model for the study of sleep-memory
relationships
Although the positive role of sleep for memory consolidation of
material learned before sleep (either procedural or declarative) has
been largely explored in the frame of the so-called ‘‘sleep effect’’
hypothesis (e.g.,
53,54
for extensive reviews) and is widely accepted
today, there is still an intense scientific debate on which sleep- or
memory-related factors are crucial for the effect.
Amongst these factors the role of a minimum sleep duration has
never been fully clarified.
54
Obviously, nap studies would be the
natural model to study the time component of sleep for memory,
but surprisingly few studies have addressed the existence of such
a ‘‘nap effect’’. Another concern is whether sleep at different
circadian phases has the same impact on memory. Likewise, rather
few (and controversial) studies have addressed this issue.
55,56
Thus,
it is somewhat premature to predict whether daytime naps could
be as effective in favouring memory consolidation as normal night
sleep. This still open question of the ‘‘nap effect’’ is reviewed in
detail below, and in the ‘‘Research Agenda’’ section.
It is important to recognize that daytime naps can work as
a powerful experimental model to elucidate mechanisms through
which sleep supports memory processes. In fact, experiments using
daytime naps on memory can avoid some of the drawbacks which
affect studies of nighttime sleep. First, if naps facilitate memory,
experimental designs can use multiple naps during the day to
better understand the role of the circadian factor. Second, day-time
nap protocols may avoid severe sleepiness, which is a factor partly
hampering the validity of studies on sleep and memory when the
control group has received no night sleep. Differences of memory
recall may indeed depend on a reduction of the control group’s
recall due to sleepiness, rather than to an increase of the sleepers’
thanks to sleep. Instead, it is much easier to maintain comparable
levels of alertness when comparing, at daytime, subjects who take
naps with subjects who are ‘‘normally’’ awake and who most
probably will not experience any significant vigilance reduction.
G. Ficca et al. / Sleep Medicine Reviews 14 (2010) 249–258252

Third, as clearly explained by Backhaus and Junghanns,
57
sleep
effect on memory consolidation can be more confounded during
night sleep than during naps by hormone secretions, especially
cortisol and GH, influencing memory encoding and recall. Finally,
the peculiar sleep architecture of daytime naps is particularly
adequate to give hints as to the role of sleep organization (i.e., the
presence of uninterrupted NREM-REM cycles), which has been
proposed to be a crucial factor for memory consolidation.
54,58
In
fact, about half of the daytime naps contain a full sleep cycle, with
a short REM episode whose presence is linked to a certain
minimum amount of preceding slow wave sleep (SWS).
59
Any effect
on memory of this kind of naps is likely to depend on other factors
than the absolute amount of sleep states, and their percentage out
of total sleep time (TST), which are overall quite small.
Effects of daytime naps on procedural memory
Most data on naps and memory processes have been produced
by administering procedural tasks.
The study which brought new interest to the ‘‘nap effect’’ was
the one by Mednick et al.
60
at the beginning of the decade, who
showed how the habitual perceptual deterioration across time on
a visual discrimination task (VDT) may be prevented by taking
a nap. Deterioration over consecutive sessions was effectively
counteracted by a nap (at 14:00 h) between sessions. The authors
found that the performance at this procedural memory task was
dependent on the duration of the nap: a shorter (30-min) nap
seems to prevent the habitual deterioration across the last two
sessions, allowing the maintenance of performance, whereas
a longer (60-min) nap reverses this impairment and improves
subsequent performance. A significant difference was found in SWS
and REM sleep amount in the short versus the long naps and,
interestingly, the impact of memory consolidation processes on
sleep structure was also suggested by the increase of the amount of
SWS and REM sleep during experimental compared with baseline
naps.
Further support to the role of naps on procedural memory was
given by a later work of the same group,
61
reporting that naps
including both SWS and REM led to performance enhancement,
whereas naps containing only SWS only prevented deterioration
across sessions. Discussing the results of this study, the authors
proposed that the amplitude of the nap-dependent improvement
on the VDT is similar to the one they had previously seen after an
eight hour sleep night.
62
They suggested that SWS may be neces-
sary to stabilize performance whereas REM sleep may facilitate an
actual performance improvement.
Backhaus and Junghanns
57
recently reported day-time naps to
favor procedural motor learning, as indexed by performance at
a mirror-tracing task. Cajochen et al. showed positive effects of naps
on procedural memory consolidation in a serial reaction time task
(SRT), in the context of a circadian protocol.
63
Korman et al.,
64
after
training subjects on a finger-to-opposition sequence-learning task,
showed that an interfering reversed sequence affects the overnight
gain normally observed at this procedural task. However, a post-
training 90-minute nap is capable of reversing this interference
effect, resulting in a larger performance gain. Finally, a very recent
study
65
has shown the benefit of a midday nap (60–90 min) on
motor memory consolidation. This clear memory enhancement in
the nap condition correlated with sleep spindle density and power
at specific regional sites (centroparietal, as expected given the areas
involved in the motor task).
The only evidence in disagreement with the aforementioned
results came from a study by Tucker et al.,
66
assessing both
procedural and declarative performance after a short nap (about
50-min duration) administered at 13:00 h, which did not find any
significant difference between sleep and wakefulness at a proce-
dural task. Methodological factors may explain this discrepancy.
Tucker et al.,
66
utilized duration of naps shorter than those used in
other studies, with accompanying relevant differences in sleep
architecture, e.g., absence of REM sleep. Furthermore, Tucker et al.
adopted a very long retention interval (five hours between the
baseline recall test and the delayed cued recall test), different from
previous studies where recall was requested after a brief interval
from awakening, just enough to permit sleep inertia dissipation.
Interestingly, Milner et al.
67
found that although motor learning
was consolidated in a brief nap and was associated with stage 2
spindles, this occurred only for those subjects who habitually take
naps, thus introducing the theoretical issue of distinguishing nap
effects in habitual and non-habitual nappers.
Effects of daytime naps on declarative memory
Some notable evidence of a ‘‘nap effect’’ has been produced for
declarative memory. The first such study was conducted by Schoen
and Badia.
68
In their study, a 2-hour nap was administered to the
subjects after learning non-meaningful (nonsense syllables) and
meaningful (short stories) material. The protocol included two
subgroups, taking a nap respectively at 7:00 h and at 15:00 h, and
a control group spending the 2-hour retention period awake.
Placing the experimental naps either early in the morning or late in
the afternoon was intended to detect a time-of-day effect, and
should have also allowed to make assumptions on the role of sleep
stages, because, due to circadian and homeostatic reasons, REM
sleep is more prevalent in early morning naps, while SWS is more
prevalent in afternoon naps. Results showed a better recall for all
kinds of verbal material in both nap conditions compared with
wakefulness, with no difference in the recall between morning and
afternoon naps.
The relationship between declarative memory processes and
daytime naps has been readdressed only recently, some twenty
years after Schoen and Badia’s work. Schabus et al.
69
studied word-
pair association performance using a cued recall procedure, before
and after a 60-min nap at 14:00 h. Subjects were later divided in
two groups according to the composition of sleep stages: nappers
with or without SWS. No REM-nap group was included because the
short nap duration (50-min average) did not permit the REM sleep
occurrence. Results indicated that only those nappers with SWS in
their sleep showed an improvement at the declarative memory task
after the nap. Quantitative electroencephalogram (EEG) analysis
showed increased theta activity during those naps which were
associated with memory improvement.
Muto et al.
70
also evaluated the recall of non-associated paired
words after a daytime nap: after an immediate free recall of verbal
material, subjects took a 2-hour nap (at 14:00 h). Successive recall
tests were carried at awakening and at a further point in the early
evening (around 19:00 h). Results showed a preventive effect of
naps against the loss of the material clearly going on in the wake
condition. Interestingly, the authors, by comparing those naps
containing a minimum amount of REM sleep (10 min) to those
without REM sleep, found that only the naps containing REM were
linked to an actual improvement of memory performance. Since
the amount of REM sleep as a fuction of total sleep time was very
small, the authors concluded that the role of REM sleep for the post
nap improvement was more likely linked to the completion of the
sleep cycle than to its presence per se. This intriguing result needs
to be replicated with a larger sample, and a study employing
a larger sample is currently in preparation by the same research
group.
In the aforementioned study by Tucker et al.,
66
unlike the
findings for procedural memory, subjects taking a nap improved
G. Ficca et al. / Sleep Medicine Reviews 14 (2010) 249–258 253

more at the declarative memory task than those subjects who
stayed awake. Moreover, a positive though non-significant corre-
lation was observed between improvement on the verbal task and
minutes of SWS, whereas no other sleep parameter appeared to
correlate with declarative memory. Thus, the authors were basi-
cally replicating data about the purported role of non-REM(NREM)
sleep for declarative memory processing.
Recently Lahl et al.
71
showed that free word recall was better
after a 60-min retention interval spent asleep rather than awake.
Differently from previous research,
69,70
no sleep parameters
correlated with recall scores. In a second experiment, the authors
tried to examine the effects of an ultra short nap (mean
TST ¼6.3 min) or a longer nap (mean TST ¼35.8 min) on verbal
material consolidation. The results indicate that even the ultra
short nap is sufficient for a better recall when compared with
wakefulness. These data are interpreted in light of a possible
facilitating effect of the mere sleep onset on memory consolidation
processes, independent of sleep stages or sleep duration. This
hypothesis had already been put forward by Tietzel and Lack
72
regarding the effect of sleep onset on cognitive performance
assessed through symbol-digit substitution and letter-cancellation
tasks.
Schmidt et al.
73
investigated the effects of declarative learning
on sleep EEG activity changes during daytime sleep, under
controlled homeostatic and circadian conditions. Unrelated verbal
material was constructed with the aim of obtaining two word lists
differing in the level of concreteness, resulting in an easier and
a more difficult associative encoding condition. Immediate and
delayed cued recall were assessed respectively pre and post a 4-
hour daytime sleep. Relative to a control condition, the authors
reported an increase of the power density in the spindle frequency
range and of the density of low-frequency sleep spindles. This
increase was observed after the difficult encoding condition but not
after the easy one. Moreover EEG spectral activity changes posi-
tively correlated with memory performance changes between pre-
and post-nap test sessions. The authors concluded that the nature
of the declarative material is a determiningt factor for sleep
parameter changes.
Napping, sleep and health in older adults
The prevalence of daytime napping increases with advancing
age.
74,75
Many factors are likely to contribute tothis, including age-
dependent changes in nighttime sleep and circadian rhythms,
co-morbid illnesses and lifestyle factors. However, the impactof the
napping of older adults on their nighttime sleep and on their
daytime functioning and health remains to be fully determined.
Here we briefly review the epidemiology of napping, the impact of
napping on nighttime sleep, and the associations between napping
and health in older adults.
Epidemiology of napping in older adults
Although prevalence rates for napping in older adults vary
widely depending on how napping behavior is defined or measured
and the population studied, older adults consistently report more
frequent napping than younger adults.
74–76
The prevalence of
napping seems to increase with advancing age within the older
population itself, with the ‘‘older old’’ typically reporting more
frequent naps than the ‘‘younger old’’.
75,76
For example, in their
analysis of the National Sleep Foundation’s ‘‘2003 Sleep in America
Poll,’’ that focused on the topic of ‘‘sleep and aging’’ in 1506 older
adults (aged 55–84 years), Foley and colleagues found that 15%
reported regular napping (4–7 times/week), ranging in prevalence
from 10% among those 55–64 years of age to 25% among those 75–
84 years of age.
75
MaCrae et al.
77
reported that in their sample of
413 older men and women (aged 60–96 years) nearly half (47.5%)
reported regular napping (>3 times/week), while 18% reported
napping more than six times per week. Finally, a recent large multi-
center study of 8101 older community-dwelling women (mean age
77.0 years), found that approximately 11% reported napping daily,
with those reporting daily napping significantly older than those
who did not.
78
There does not appear to be a clear gender difference in napping
among older adults. While a few studies have suggested that older
men may nap more frequently than older women,
79,80
most other
studies report no such differences.
74,75,77
A limitation of the survey technique employed by studies
reviewed above is that older adults often take unplanned or unin-
tentional naps, which they may not even be aware of doing (for
example, falling asleep in the evening while reading or watching
television) and therefore underestimate their napping behavior.
This problem is illustrated by the study of Yoon and colleagues
81
who studied napping behavior in younger and older adults using
wrist actigraphy. They reported that many older adults may have
bouts of sleep during the evening but not report them as naps,
although they also recognize that actigraphy may have difficulty
distinguishing true napping from sitting quiescently while watch-
ing television or reading.
Little work has been done examining planned versus unplanned
napping. Recent preliminary work in this area has been conducted
by Vitiello and Foley
82
using the National Sleep Foundation’s ‘‘2003
Sleep in America Poll,’’ of 1506 older (aged 55–84 years) adults.
They found that 36% of the sample (n ¼515) reported napping 1–7
times/week, with half of those (50.7%) reporting planned versus
unplanned napping. Being older and unmarried, excessive daytime
sleepiness, anhedonia and drinking <4 coffees/day were all
predictors of unplanned napping. However, neither measures of
nighttime sleep complaint nor of health burden predicted
unplanned napping, suggesting, albeit counter-intuitively, that
these factors have little influence on one’s plans for napping.
Causes of daytime napping in older adults
Daytime napping may be the result of a variety of causes,
including: disturbed nighttime sleep,
83,84
age-related phase
advance of circadian rhythm,
85
co-morbid medical and psychiatric
illnesses,
75
and lifestyle factors, or some combination of these.
However, the evidence supporting the direct contribution of any of
these factors to napping is far from conclusive. This may be because
the relationships between napping and many, and possibly most, of
these possible causal factors are likely to be bi-directional, with
disturbed sleep, advanced phase of circadian rhythms, or poor sleep
habits leading to daytime napping and daytime napping, in turn,
disrupting nighttime sleep, shifting circadian rhythm or prompting
poor sleep-related behaviors.
Impact of napping on nighttime sleep in older adults
The impact of daytime napping on nighttime sleep is unclear, as
studies in this area have yielded inconsistent findings. It is
commonly assumed that daytime napping should decrease
homeostatic sleep drive during the following night, resulting in
increased nighttime wakefulness and decreased sleep efficiency,
which would result in being tired the next day, increasing the
likelihood of napping that day with the resulting nighttime impact,
maintaining this pattern through a positive feedback process.
Several studies provide support for this conceptualization that
daytime napping can negatively impact nighttime sleep in older
adults. Monk and colleagues
86
found that a daytime nap shortened
G. Ficca et al. / Sleep Medicine Reviews 14 (2010) 249–258254

nighttime sleep, decreased sleep efficiency and led to earlier
morning awakening among nine healthy olderadults. Yoon et al.
81
observed that healthy older adults who were evening nappers had
shorter sleep durations and awakened earlier in the morning
compared to older adults who did not nap in the evening. Campbell
et al.
87
examined the impact of an afternoon nap on subsequent
nighttime sleep quality in healthy older adults concluding that
napping had little effect, although they reported a significant 40%
increase in sleep latency and non-significant increases in nighttime
wakefulness following napping compared to sleep following an
afternoon sedentary period. Liu and Liu
88
in their study of 1820
older Chinese adults reported that older age, poor perceived health,
being female, being unmarried and frequent napping were all risk
factors for insomnia.
However, other studies have not supported this relationship
between napping and sleep quality. Metz and Bunnell
76
surveyed
132 older adults (ages 58–95) and reported that no significant
relationship was observed between any reported napping
dimension and indicators of nocturnal sleep difficulty. Rather age
had the greatest effect on napping, with older subjects taking
more frequent and longer naps; although a non-significant trend
suggested that the duration of naps might be associated with
increased difficulty initiating nocturnal sleep. Bliwise
89
examined
sleep quality and related factors in 36 healthy older women, self
identifying as good or poor sleepers. While poor sleepers reported
longer sleep latencies, less total sleep time, more non-restorative
sleep, and more daytime fatigue than did good sleepers, no
differences were found between good and poor sleepers in their
number of daily naps. Mallon and Hetta,
80
reporting on the sleep
habits and difficulties of 876 older Swedes observed that daytime
napping was common, but not related to poor sleep. This lack of
a relationship was confirmed by Hsu
90
who also found no
correlation between naps and quality of sleep among 80
community-dwelling Chinese elders. Finally, Foley et al,
75
reporting on napping in 1506 older adults, observed that frequent
napping was associated with excessive daytime sleepiness,
depression, nocturia and pain, but not with complaints of poor
nighttime sleep.
Napping and health in older adults
Napping has been reported to be associated with increased risk
of mortality, cardiovascular disease, falls and cognitive impairment
in older adults. A number of studies have reported that napping is
related to increased risk for mortality (e.g.,
91–95
), whereas the
association between napping and cardiovascular disease is unclear,
with some studies reporting an association (e.g.,
87,94
), but not all,
(e.g.,
96,97
) including a very recent large-sample study, which
controlled for pre-existing co-morbidites.
98
Brassington et al.
99
and Stone et al.
78
have both reported that napping is associated
with increased risk for falls, with Stone et al. also reporting napping
was associated with an increased risk of hip fracture. Napping may
also be associated with increased risk of cognitive impairment,
particularly compromised executive function in older women.
100
This last observation would seem at odds with a previous section of
this review which noted the positive impact of napping on cogni-
tion, particularly memory. However, it is important to distinguish
the potential beneficial effect of experimentally imposed naps on
memory from the cognitive impairment associated with the
frequent napping of older adults that has been shown to be asso-
ciated with increased morbidity
77,78,82,88,99
and mortality.
91–95
This
contrast clearly illustrates the complexity of the interrelationships
among napping, health and cognition, which have yet to be fully
delineated.
Concluding remarks
Evidence from both laboratory experiments and ‘‘field’’ studies
appears to indicate an overall beneficial role of napping for neu-
rocognitive functions. First, all studies to date demonstrate that
correctly timed split-sleep either had a positive effect or no effect
on subsequent neurobehavioral performance supporting the
hypothesis that the restorative effects of sleep on performance may
be mantained when splitting the overall sleep episode into multiple
naps. However, whether or not performance differences between
consolidated and split-sleep schedules were observed, all of the
experiments that measured sleep with PSG reported significant
differences in the architecture of sleep stages in terms of the
proportion of slow wave sleep and REM.
19,21–23
This raises the
theoretical question of why split-sleep schedule induced differ-
ences in sleep architecture do not translate into differences in
subsequent neurobehavioral performance.
This result has important implications about the operational
feasibility of split-sleep schedules because, even if performance is
not enhanced by split-sleep, it provides moreflexibility to schedule
sleep in work environments that involve restricted nocturnal sleep
due to critical task scheduling.
The majority of nap studies in the work environment clearly
lead to the same conclusion. In fact, the literature on shift workers
shows that the alertness- and performance-enhancing effects of
naps during both night and afternoon shifts can be quite dramatic.
For instance, it appears that a daytime nap as short as 10-min can
improve alertness and performance for about 2.5 h in the face of
prior sleep loss, and for almost 4 h if preceded by normal sleep.
However, it remains to be determined what the best nap duration is
to achieve the most effective alerting effects, and to what extent
this might be modulated by individual variables. Moreover,
whether and how different cognitive functions (attention, working
memory, higher cognitive functions such as decision making and
planning) are differentially influenced by napping remains an open
question.
The overview pattern of results also seems to point to a global
beneficial effect of daytime naps for memory recall. Quite
surprisingly, and differently from what has been reported in the
‘‘night sleep effect’’ literature, almost all authors report a beneficial
‘‘nap effect’’ for both procedural and declarative tasks. However,
the effect strength ranges from quite dramatic changes, even
including the actual improvement of memory performance at
awakening,
61,70
to less relevant modifications, usually limited to
a reduction of the forgetting/deterioration rate,
60,67,70
which
seemingly depend on numerous factors, either related to sleep or
memory. In fact, further studies are needed to better understand:
a) what sleep features are crucial for the ‘‘nap effect’’ on memory
consolidation, clarifying the role of nap duration, sleep states
amount, sleep continuity and organization, and whether
a memory-enhancing process is triggered by sleep onset per se;
and b) whether and to what extent each memory task is differ-
entially responsive to such a nap effect.
It is also worthwhile underscoring that the effects of naps on
health and on the whole spectrum of cognitive functions might be
different for habitual compared to non-habitual nappers.
As for the relationships between napping and age, we have not
examined the effects of naps on babies’ and children’s cognitive
processes and learning abilities, since no data have been produced
on this topic apart from a pioneering study by Gomez et al.
101
Instead, we focused on napping behavior in the elderly. The state-
of-the-art concerning napping in older adults was summarized in
a recent Journal of the American Geriatrics Society editorial.
102
The
prevalence of spontaneous napping increases with age in adults.
This increase is likely the result of increases in nighttime sleep
G. Ficca et al. / Sleep Medicine Reviews 14 (2010) 249–258 255

disturbances, phase advance of circadian rhythms, co-morbid
medical and psychiatric illnesses and poor sleep habits. Napping
does not seem to be associated with nighttime sleep quality but
rather with excessive daytime sleepiness and medical and psychi-
atric co-morbidities. Frequent, unplanned, longer daytime naps in
older adults have the potential to negatively impact nighttime sleep
quality and may be associated with significant negative health
consequences such as increased risk of morbidity, cardiovascular
illness, falls and cognitive impairment. However, it is possible that
brief planned naps may be of benefit to the function of healthy
older adults (e.g.,
84
), and perhaps even older adults in poorer
health.
31
We can conclude by saying that research on naps is at a very
early stage and that there is much more to learn about napping.
As suggested by Vitiello,
102
the field needs to move away from
relatively simplistic assessments of napping to more nuanced
assessments employing more elaborate self-reports paralleled by
objective assessment techniques, e.g., actigraphy, which will allow
for a better appreciation of the complexity of napping behavior.
Such techniques would be best employed in large, representative
samples which include both nappers and non-nappers. Only once
such quantitative assessment techniques are employed longitu-
dinally in large representative samples will a better under-
standing of the prevalence, antecedents and consequences of
napping across the full spectrum of sleep and health patterns be
possible.
Acknowledgements
Dr. Vitiello is supported by PHS grants: AG025515, AG031126,
MH072736, NR001094, and CA116400.
We wish to thank Dr. Francesca Conte and Dr. Ciro Della Monica
for their precious help in revising the manuscript.
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Correctly timed split-sleep, shown to have positive
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wake schedules in work environments that involve
restricted nocturnal sleep due to critical task scheduling.
Napping is an effective important countermeasure to
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Frequent, unplanned, longer daytime naps in older
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morbidity, cardiovascular illness, falls and cognitive
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However, brief planned naps may be of benefit to the
function of healthy older adults, and perhaps even older
adults in poorer health.
Research agenda
How and to what extent the neurocognitive effects of
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Further studies are needed if we aim to understand the
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It is still to be understood what sleep features are crucial
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Lab studies could seek the effects of naps on specific
higher cognitive functions using ad-hoc paradigms and
tasks.
Research on naps and cognition should include the
study of oneiric activity (i.e., dream features) during
napping.
The cognitive effects of napping at early ages should be
explored, because this might be of interest with respect
to learning processes and school performance.
More elaborate self-report paralleled by objective
assessment techniques, such as actigraphy, which allow
for a better appreciation of the complexity of napping
behavior, should be employed in large, representative
samples of older adults which include both nappers and
non-nappers.
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