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the morning on day 1, tested at 19:00 that evening, and then retested at
9:00 the next morning. In the task, a target screen is briefly flashed,
consisting of a 19 × 19 grid of horizontal bars containing three diago-
nal bars and a central fixation target. Then, after a variable-length
interstimulus interval (ISI), a mask consisting of both horizontal and
diagonal bars appears. The diagonal bars, located in the lower-left
visual quadrant, form either a horizontal row or vertical column. After
each trial, subjects report whether the diagonal bar array was in a row
or column arrangement, as well as whether the central fixation target
was an "L" or "T." Performance is defined as the threshold ISI required
for a subject to be 80% accurate in the row/column discrimination.
Two experimental groups took naps at 14:00 (group 1: total sleep time,
59.3 ± 6.4 (mean ±s.d.); SWS, 20.2 ± 2.0; REM, 4.2 ± 2.2 min; group 2:
total, 96.3 ± 6.3; SWS, 47.2 ± 5.8; REM, 25.6 ± 4.1 min), while ‘no-nap’
control subjects went about their normal day without midday sleep.
All subjects reported sleeping an average of 7.6 ± 2.1 h on the night
before day 1 and 7.5 ± 1.2 h on the night before day 2. Improvement
was measured as a decrease in threshold ISI from each subject’s base-
line threshold (at 9:00 on day 1)
9
.
No-nap control subjects showed the expected deterioration in per-
formance at 19:00 (13.7 ms longer ISI threshold, P = 0.06, Fig. 1), and
performed significantly worse than the nap groups (P = 0.02). The
deterioration in performance from training to first retest in this group,
measured over an 8-h interval, was similar to that seen in our prior
–15
–10
–5
0
5
10
15
Threshold improvement (ms)
No-nap
–SWS
–REM
(
n
= 28)
60-min nap
90-min nap
(
n
= 13)
(
n
= 2)
(
n
= 13) (
n
= 17)
+SWS
–REM
+SWS
+REM
Figure 1 Same-day improvement in no-nap, 60-min nap and 90-min nap
groups, with and without REM and SWS. Left, no-nap group shows
deterioration at 19:00 from baseline test at 9:00. Center, performance
after naps with SWS but without REM shows neither deterioration nor
improvement. Right, naps with SWS and REM led to significant
improvement. Only two subjects in the 90-min nap group showed no REM.
Experiments were approved by the Committee on the Use of Human
Subjects of Harvard University. Informed written consent was obtained from
all subjects.
BRIEF COMMUNICATIONS
Sleep-dependent learning: a nap
is as good as a night
Sara Mednick
1
,Ken Nakayama
1
& Robert Stickgold
2
The learning of perceptual skills has been shown in some cases
to depend on the plasticity of the visual cortex
1
and to require
post-training nocturnal sleep
2
. We now report that sleep-
dependent learning of a texture discrimination task can be
accomplished in humans by brief (60– 90 min) naps containing
both slow-wave sleep (SWS) and rapid eye movement (REM)
sleep. This nap-dependent learning closely resembled that
previously reported for an 8-h night of sleep in terms of
magnitude, sleep-stage dependency and retinotopic specificity,
and it was additive to subsequent sleep-dependent
improvement, such that performance over 24 h showed as much
learning as is normally seen after twice that length of time. Thus,
from the perspective of behavioral improvement, a nap is as
good as a night of sleep for learning on this perceptual task.
A wide variety of learning processes in both humans and animals
requires extended, post-training sleep (for review, see ref. 3).
Depending on the nature of the task, differing stages of sleep con-
tribute to this sleep-dependent consolidation process. For the most
part, studies in humans have investigated the benefits of nocturnal
sleep
3
.Research on behavioral effects of napping has found improve-
ment in alertness, productivity and mood
4,5
, as well as restoration of
perceptual deterioration
6
.But whether relatively brief daytime naps
can produce learning is not known.
Improvement on visual perceptual tasks can occur over periods of
minutes to hours
7
, as well as over extended periods of days
2
, and
remain stable for months
8
.For a visual texture-discrimination task in
which subjects must rapidly discriminate the orientation of a target
embedded in distractors
9
, initial improvement is seen over the first few
minutes of training
10
, and slower improvement is seen over subse-
quent nights of sleep
2,11
.
Both SWS
12
and REM
13
are implicated in a two-stage model of noc-
turnal consolidation, with overnight improvement being retinotopi-
cally specific and highly correlated with the product of the amount of
early-night SWS and late-night REM sleep
11
.Repeated testing within a
day on this task leads to a retinotopically specific deterioration in per-
formance
6
,which is reversed by 60-min midday naps rich in SWS. But
these naps have relatively little REM sleep, and fail to produce signifi-
cant improvement over baseline performance. Similar deterioration
without sleep is also seen at night
12
.
We now report that 60- and 90-min naps containing both SWS and
REM facilitate learning on this texture-discrimination task, in a man-
ner similar to that seen after nocturnal sleep.
Subjects were trained on the texture discrimination task at 9:00 in
1
Psychology Department, Harvard University, 33 Kirkland Street, Cambridge, Massachusetts 02138, USA.
2
Department of Psychiatry at Massachusetts Mental Health
Center, Harvard Medical School, Boston, Massachusetts 02115, USA. Correspondence should be addressed to S.M. (smednick@post.harvard.edu).
NATURE NEUROSCIENCE VOLUME 6
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JULY 2003 697
© 2003 Nature Publishing Group http://www.nature.com/natureneuroscience
BRIEF COMMUNICATIONS
study, which showed 13.8 ms deterioration over a 2-h interval
6
.This
suggests that stimulus exposure rather than inter-test interval pro-
duces deterioration in texture discrimination, and that sleep, rather
than time, is required to reverse this perceptual deterioration
6
.
When tested at 19:00, the 90-min nap group showed significant
improvement (8.4 ms, P = 0.008), whereas the 60-min nap group
showed marginal improvement (4.4 ms, P = 0.07). Mindful of our
hypothesis that both SWS and REM may be necessary for learning in
naps, we divided the 60-min nap group, all of whom had SWS, into
subjects with and without REM and found that 60-min naps with both
SWS and REM produced significant improvement (10.0 ms, P = 0.004,
Fig. 1 right). In contrast, 60-min naps with SWS but not REM showed
no improvement (–1.1 ms, P = 0.72, Fig. 1 center) and significantly less
than seen in the SWS + REM group (P = 0.01). The 90-min nap group
showed similar results (Fig. 1,white bars). When subjects with SWS +
REM naps from the 60- and 90-min groups were combined, improve-
ment correlated significantly with the product of the amount of SWS
and REM sleep (stepwise regression, r = 0.37, P = 0.01), similar to find-
ings for nocturnal sleep
11
.The additive effects of SWS and REM seen
in these naps are also similar to those for early and late night sleep
reported previously
12
.In addition, the amount of improvement did
not differ significantly from that seen in our previous study of
overnight improvement
2
(9.7 vs. 11.9 ms; P = 0.5). Thus, nap-
dependent improvement showed the same magnitude and sleep-stage
dependency as did overnight improvement
2,11
(although of lower
magnitude than that reported for overnight improvement by oth-
ers
12,13
). Naps with SWS but not REM reversed the deterioration but
did not produce actual improvement, whereas naps with both SWS
and REM did both, suggesting that SWS may serve to stabilize per-
formance, and REM may actually facilitate performance improve-
ment.
We t ested the retinotopic specificity of nap-dependent learning by
training subjects at 9:00 in one visual quadrant (lower left or lower
right, counterbalanced) and then retesting them in the contralateral
quadrant at 19:00, with a 90-min nap at 14:00. Training plus a nap had
no significant effect on the untrained quadrant (P > 0.2), indicating
that nap-dependent learning has a retinotopic specificity similar to
that reported for overnight improvement
9
, as well as for same-day
deterioration
6
, and suggesting that the beneficial effects of a 90-min
midday nap are localized to primary visual cortex
9
.
Nap-dependent improvement was not at the expense of subsequent
nocturnal improvement. On the contrary, when subjects were retested
the next morning, the 90-min nap group showed an additional 9.7 ms
of improvement (24-h total, 18.1 ms, P < 0.0001), and greater
improvement than the no-nap group (Fig. 2, P = 0.03).
The no-nap group showed deterioration at 19:00, but normal levels
of improvement on day 2 (7.8 ms; Fig. 2, no-nap). This improvement
did not differ significantly from a second (24-h) control group trained
at 9:00 and retested 24 h later without an intervening test on the
evening of day 1 (P > 0.4; Fig. 2, 24-h control).
The nap group actually showed 50% more improvement over a
period of 24 h than the 24-h control group (18.1 vs. 11.8 ms, P = 0.07).
Indeed, 24-h improvement in the nap group was as great as that previ-
ously reported
2
after two nights of sleep (Fig. 2, 48-h control; 18.1 ms
vs. 17.5 ms; P > 0.99). Taken together, these findings indicate that a 90-
min nap can produce as much improvement as a night of sleep, and a
nap followed by a night of sleep provides as much benefit as two nights
of sleep.
These findings show that naps can lead to improved performance
on a texture discrimination task similar to previously reported learn-
ing after a full night of sleep
2
, in terms of magnitude, retinotopic
specificity and dependence on both SWS and REM. Similar improve-
ment has been reported after 192 min of early-night sleep
12
,where
subjects averaged 74 min of SWS and 24 min of REM—an amount of
REM similar to the 25.6 min found in our 90-min nap group.
Finally, napping can significantly enhance the improvement that
develops over 24 h. Thus, a nap can not only ameliorate experience-
dependent perceptual deterioration, but can also facilitate the learning
process that results from an hour spent training on a visual texture dis-
crimination task.
ACKNOWLEDGMENTS
This research was supported by grants from the US National Institutes of Health
(MH 48832, DA 11744 and NS 26985) and the Air Force Office of Scientific
Research (83-0320).
COMPETING INTERESTS STATEMENT
The authors declare that they have no competing financial interests.
Received 18 March; accepted 16 May 2003
Published online 22 June 2003; doi:10.1038/nn1078
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Day 3
48-h
Control
Threshold improvement (ms)
25
20
15
10
5
0
Day 2
Nap
No-nap
24-h
Control
Figure 2 Improvement for nap and no-nap groups. Left, improvements 24 h
after training for the no-nap group’s second retest, the 24-h control group’s
first retest, and the 90-min nap group’s second retest, all at 9:00 on day 2.
Dotted line shows nap group’s improvement on day 1. Right, improvement
48 h post-training with no nap. Data for 48-h controls are from ref. 2. Data
for 24-h controls were combined with previously published data
2
, which
were not significantly different from the present sample.
© 2003 Nature Publishing Group http://www.nature.com/natureneuroscience
