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RESEARCH ARTICLE
Future-relevant memories are not selectively
strengthened during sleep
Jennifer E. AshtonID
1
*, Scott A. Cairney
1,2
1Department of Psychology, University of York, York, United Kingdom, 2York Biomedical Research
Institute, University of York, York, United Kingdom
*jennifer.ashton@york.ac.uk
Abstract
Overnight consolidation processes are thought to operate in a selective manner, such that
important (i.e. future-relevant) memories are strengthened ahead of irrelevant information.
Using an online protocol, we sought to replicate the seminal finding that the memory benefits
of sleep are enhanced when people expect a future test [Wilhelm et al., 2011]. Participants
memorised verbal paired associates to a criterion of 60 percent (Experiment 1) or 40 percent
correct (Experiment 2) before a 12-hour delay containing overnight sleep (sleep group) or
daytime wakefulness (wake group). Critically, half of the participants were informed that
they would be tested again the following day, whereas the other half were told that they
would carry out a different set of tasks. We observed a robust memory benefit of overnight
consolidation, with the sleep group outperforming the wake group in both experiments. How-
ever, knowledge of an upcoming test had no impact on sleep-associated consolidation in
either experiment, suggesting that overnight memory processes were not enhanced for
future-relevant information. These findings, together with other failed replication attempts,
show that sleep does not provide selective support to memories that are deemed relevant
for the future.
Introduction
It is now well established that sleep supports the consolidation of newly acquired declarative
memories [1–4]. Sleep consistently results in better memory retention when compared to
equivalent periods of wakefulness [1,2,5–10] and these behavioural effects have been associated
with specific features of sleep (e.g. slow-wave sleep [11–13]). Contemporary models of sleep-
associated consolidation suggest that newly formed memories are reactivated during sleep,
prompting their migration from hippocampus to neocortex for long-term storage [1,2,13,14].
However, new memories do not all profit equally from sleep (i.e. some memories benefit more
from sleep than others), and the factors that determine to what extent a memory will benefit
from overnight consolidation are not fully understood.
There is growing evidence to suggest that sleep offers special protection to salient informa-
tion. For example, the benefits of sleep are enhanced for memories that are considered emo-
tionally negative [15–19], that are associated with monetary reward [20] or that are deemed
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OPEN ACCESS
Citation: Ashton JE, Cairney SA (2021) Future-
relevant memories are not selectively strengthened
during sleep. PLoS ONE 16(11): e0258110. https://
doi.org/10.1371/journal.pone.0258110
Editor: Bradley R. King, University of Utah, UNITED
STATES
Received: June 9, 2021
Accepted: September 18, 2021
Published: November 4, 2021
Copyright: ©2021 Ashton, Cairney. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Study data are
available on the Open Science Framework database
(osf.io/3nqpd).
Funding: This work was supported by a Medical
Research Council Career Development Award (MR/
P020208/1) to S.A.C. The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
relevant for the future [21–23]. The perceived relevance of newly learned information can be
manipulated experimentally by controlling test expectancy; that is, whether or not an individ-
ual expects their memory to be assessed. In their seminal study, Wilhelm et al. [24] trained par-
ticipants on a declarative memory task before a delay containing sleep or wakefulness.
Crucially, half of the participants were informed that they would be re-tested in a later session,
whereas the other half were told that they would complete a different set of tasks. Among par-
ticipants that slept (but not those who remained awake), declarative memory retention was
better for individuals who expected a test, as compared to those who did not, suggesting that
sleep had selectively strengthened future-relevant information.
However, other findings concerning the selective influences of sleep have been somewhat
contradictory. A number of studies have failed to observe a preferential benefit of sleep on
emotionally negative relative to neutral memories [6,25–30], and others have observed no
impact of monetary reward on sleep-associated consolidation [31–33]. In recent work, knowl-
edge of a future test had no impact on the overnight retention of declarative or nondeclarative
memories [34], and failed to influence the neural correlates of memory retention after sleep
[35]. The circumstances by which future expectations affect the salience of newly acquired
information, and the subsequent impacts on sleep-associated memory processing, therefore
remain unclear.
To address this issue, we sought to replicate the seminal findings of Wilhelm et al. [24] in
an online protocol and test the hypothesis that the benefits of sleep for memory are amplified
when newly learned information is deemed relevant for the future. In Experiment 1, partici-
pants completed verbal paired associates training (encoding and baseline test) before a
12-hour retention interval containing overnight sleep or daytime wakefulness. Critically, after
training, half of the participants were informed that they would be tested again in the following
session, whereas the other half were told to expect a different set of tasks. All participants were
re-tested 12 hours later, and again after 7 days, allowing us to study the impact of test expec-
tancy on sleep-associated consolidation and to determine the extent to which these effects are
maintained in the longer term. Because participants received feedback on their final round of
baseline testing, we expected to observe an improvement in memory performance between the
baseline and follow-up tests. Importantly, however, we also expected that the extent of this
improvement would be greater after sleep than wakefulness and, furthermore, that this sleep-
associated memory gain would be augmented for individuals who had knowledge of a future
test.
There is growing evidence to suggest that sleep favours the consolidation of weakly-
encoded relative to strongly-encoded memories [36–38]. We therefore reasoned that if base-
line performance levels are too high, then knowledge of a future test might not augment sleep-
associated consolidation. We tested this possibility in our second, pre-registered experiment
(osf.io/3k6ej), by adjusting our paired associates memory paradigm to reduce pre-sleep
performance.
Experiment 1
Methods
Participants. Two-hundred and thirty adults (163 women, mean ±SD age: 24.89 ±3.66
years) were recruited online via Prolific (https://app.prolific.co/) and reported to be living in
the UK with English as their first language. On the days of the experiment, participants were
asked to abstain from alcohol and avoid taking naps. Informed consent was obtained from all
participants in line with the Research Ethics Committee of the Department of Psychology at
the University of York. Only participants who expected/did not expect a memory test in line
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with their respective condition were included in the final analysis (e.g. individuals in the unex-
pected condition who reported to have expected a follow-up test after the baseline test were
excluded; see Follow-up questionnaires below). The number of participants excluded from
each condition was: sleep-expected n= 12, sleep-unexpected n= 19, wake-expected n= 10,
wake-unexpected n= 25. A further two participants were excluded; one for reporting to have
used pen and paper to memorise the stimuli, and another who reported napping during the
day of the study. This resulted in a final sample of 162 participants (117 women, mean ±SD
age: 25.04 ±3.74 years) who were allocated to one of four conditions: sleep-expected (n= 42),
sleep-unexpected (n= 39), wake-expected (n= 50) or wake-unexpected (n= 31). Assignment
to the expected and unexpected conditions was determined by the time that participants
signed up for the study (AM or PM), which alternated between testing days. Evening sign ups
were always allocated to the sleep group and morning sign ups were always allocated to the
wake group.
Our sample size was calculated using an effect size reported in Wilhelm et al. [24]. The
effect of interest (d= 0.86) was derived from a t-test comparing memory retention after sleep
in individuals who did and did not expect a test. We determined that a minimum sample size
of n= 120 (n= 30 per condition) would be necessary for 95% power (one-tailed, alpha = .05)
to detect an effect of this magnitude. Data collection continued until our desired sample size
was met, with each participant providing a full and usable data set.
Procedure. A schematic of the study is shown in Fig 1. All participants completed verbal
paired associates training (encoding and baseline memory test) in the morning (between 7am
and 9am; wake groups) or evening (between 7pm and 9pm; sleep groups). Participants then
returned 12 hours and 7 days after encoding to complete follow-up tests.
Critically, after the baseline test, half of the participants (in both the wake and sleep groups)
were informed that their memory for the pairs would be tested in the next session (expected
condition), whereas the other half were told they would carry out a different set of tasks (unex-
pected condition).
Encoding and baseline assessment. The paired associates task required participants to learn
40 semantically-related word pairs (e.g. horizon–sun). On each encoding trial, a randomly
selected word pair was presented in the centre of the screen for 5 s, followed by an inter-stimu-
lus interval of 200 ms. Participants were instructed to commit each pair to memory by imagin-
ing a scenario in which the words were interacting (for example, one might imagine the sun
coming up over the horizon in the example above). Baseline memory performance was
assessed immediately after encoding using a cued-recall procedure (for all 40 word pairs). On
each trial, the first word (cue) of a randomly selected pair was presented on the centre left of
the screen and participants were required to type the associated second word (target). Partici-
pants had 10 s to provide each response and submitted their answer by pressing ‘enter’ on the
Fig 1. Study procedure. All participants completed a verbal paired associates task (encoding and baseline test) in the
morning (wake groups) or evening (sleep groups). Afterwards, participants were informed that their memory for the
word pairs would be tested in the next session (expected condition) or were told to expect a different set of tasks
(unexpected condition). All participants were tested again 12 hours and 7 days after encoding.
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keyboard. Following each response, and irrespective of accuracy, the cue and target words
were displayed together for 2 s. Participants were instructed to use this feedback to relearn any
pairs that they may have recalled incorrectly. This cued-recall procedure was repeated for a
second time if participants had not reached a criterion of 60% correct responses in the first
baseline test (all 40 word pairs were re-tested). Participants completed the test a maximum of
two times (55% of participants completed a second test). The proportion of participants who
completed a second test did not differ between the sleep and wake groups, X
2
(1, N= 162) =
0.62, p= .430.
Expectancy manipulation and retention interval. Following the baseline memory assess-
ment, participants in the ‘expected’ groups were told that they would be retested in the next
session, whereas participants in the ‘unexpected’ groups were told they would perform a differ-
ent set of tasks. To avoid active rehearsal of the word pairs, all participants completed a puzzle
matching game for 5 minutes before finishing the session. Participants then entered a 12-hour
retention interval, which took place across the day (wake groups) or night (sleep groups). Par-
ticipants in the wake groups followed their usual daytime routine and abstained from taking
naps. Participants in the sleep groups followed their usual home sleep routine. Participants
were not given any explicit instructions about the tasks that they would perform at the final fol-
low-up (7 days later).
Memory retrieval. Participants returned 12 hours and 7 days after the first session. During
these follow-up sessions, their memory for all 40 word pairs was retested using the same cued-
recall procedures as at the baseline session, with the exception that no feedback was provided.
Follow-up questionnaires. Participants completed questionnaires probing the amount of
sleep that they had achieved during the 12-hour delay (sleep groups), or whether they had
taken any naps (wake groups). Critically, all participants reported whether or not they had
expected to be retested on the word pairs after the baseline test. Participants in the unexpected
conditions who reported to have expected a test were excluded, as were participants in the
expected conditions who reported to have not expected a test. As outlined above (see Partici-
pants), the majority of individuals excluded on this basis were those in the unexpected condi-
tion who reported to have expected another test after the baseline test. A total of 66
participants were excluded on this basis in Experiment 1 (see Participants).
Alertness. Alertness was assessed at the beginning of each session using the Stanford Sleepi-
ness Scale (SSS [39]) and a Psychomotor Vigilance Task (PVT). During the PVT, participants
were presented with a blank grey screen. At random intervals, a red cross appeared in the cen-
tre of the screen and participants were required to press the keyboard space bar as quickly as
possible. Inter-stimulus intervals were randomly distributed from 2 to 10 s and the task lasted
for a total of 3 minutes.
Analysis. Memory retention at each follow-up test was defined as the percentage of cor-
rectly recalled word pairs, with performance on the final round of the baseline test set to 100%
[24]. For example, if a participant recalled 30 out of 40 words at baseline and then 35 out of 40
words at the 12-h follow-up, then their 12-h retention score would be ~117% ([35/30]�100).
Owing to the feedback provided during the final baseline assessment, cued-recall performance
at the follow-up tests was expected to be above 100%. Retention scores at the 12-hour and
7-day tests were applied to separate 2 (Delay: Sleep/Wake) �2 (Expectancy: Expected/Unex-
pected) ANOVAs. To assess the strength of non-significant effects, a corresponding factorial
Bayesian ANOVA was calculated for each test with the same factors as above. By convention,
the strength of evidence for the null hypothesis, in comparison to the experimental hypothesis,
is regarded as noteworthy if the Bayes Factor (BF
01
) is >3 [40].
Alertness in each session was measured using scores provided on the SSS [39]. Vigilance
was assessed by the number of attentional lapses (i.e. trials with a response time [RT] >500
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ms) made during the PVT [41]. These measures were subjected to separate 2 (Delay: Sleep/
Wake) �2 (Expectancy: Expected/Unexpected) ANOVAs at each session. All analyses were
completed in R[42] using the Rstatix [43] package.
Results
Baseline memory performance
Across all conditions, baseline memory performance (calculated as the percentage of correctly
recalled word pairs in the final baseline test) was 77% ±0.95 (mean ±SEM; see Table 3). A 2
(Delay: Sleep/Wake) �2 (Expectancy: Expected/Unexpected) ANOVA showed no group dif-
ferences at baseline (Delay: F(1,158) = 0.10, p= .320, Expectancy: F(1, 158) = 0.12, p= .726)
and no Delay�Expectancy interaction (F(1, 158) = 0.02, p= .900). Nine participants failed to
reach the performance criterion of 60% after two attempts. The lowest score among these par-
ticipants was 37.5% and the average score was 48%. Our results did not change when we
repeated the analysis without these participants (all p>.05; also see analysis of memory reten-
tion below).
Memory retention
Memory retention at the 12-hour test was better after sleep than wakefulness (F(1, 158) = 9.67,
p= .002, η
p2
= 0.06, Fig 2A). However, performance was not influenced by knowledge of a
future test (F(1, 158) = 0.63, p= .428) and there was no Delay�Expectancy interaction (F(1,
158) = 0.07, p= .786). Sleep-associated memory benefits were not therefore amplified for indi-
viduals who expected a test. In line with this interpretation, our Bayesian ANOVA yielded a
Delay�Expectancy BF
01
of 4.00, indicating moderate evidence in favour of the null hypothesis.
The same pattern of results was observed at the 7-day test (Fig 2B), with an overall benefit
of sleep on memory retention (F(1, 158) = 4.30, p= .040, η
p2
= 0.03). Knowledge of a future
test did not influence performance (F(1, 158) = 1.96, p= .163) and there was no
Delay�Expectancy interaction (F(1, 158) = 1.34, p= .249, BF
01
= 2.47).
These results remained the same when participants who did not meet the 60% baseline per-
formance criterion were removed from the analysis. Only a main effect of Delay was observed
at the 12-hour (F(1,149) = 16.93, p<.001, η
p2
= 0.10) and 7-day test (F(1,149) = 4.68, p= .032,
η
p2
= 0.03) was observed. There were no main effects of Expectancy and no Delay�Expectancy
interactions at either test (all p>.05).
To capture the change in retention between the 12-hour and 7-day tests, memory retention
at the 7-day test was re-analysed, with performance in the 12-hour test set to 100%. When ana-
lysed with the same ANOVA as described above, there were no differences between conditions
(Delay: F(1, 158) = 0.95, p= .330; Expectancy: F(1, 158) = 0.99, p= .321), and no
Delay�Expectancy interaction was observed (F(1, 158) = 2.25, p= .136).
Self-reported sleep
Participants in the sleep groups reported the number of hours that they had slept between
encoding and the 12-hour test. Mean (±SEM) hours slept was comparable between the sleep-
expected (6.58 ±0.25) and the sleep-unexpected (7.01 ±0.21) conditions (t(79) = 1.28, p=
.203). To ensure that our results were not influenced by poor sleepers (i.e. those who reported
less than 6 h of sleep) we repeated our main analyses without these participants (n = 13). The
foregoing findings were replicated, with only a significant memory benefit of sleep emerging
at the 12-hour (F(1, 145) = 10.15, p= .002, η
p2
= 0.07) and 7-day test (F(1, 145) = 4.27, p= .041,
η
p2
= .03). All other effects and interactions were non-significant (p>.05).
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No significant correlation was observed between self-reported sleep and task performance
in either the sleep-expected (12-hour test: r= 0.06, p= .686; 7-day test: r= 0.05, p= .779) or
sleep-unexpected condition (12-hour test: r= 0.20, p= .200; 7-day test: r= 0.26, p= .109).
Alertness
Alertness levels, as indicated by the SSS (Table 1) were comparable across the Delay and
Expectancy conditions at the baseline test and 7-day test (all p>.05). Interestingly, at the
12-hour test, the sleep group rated themselves as less alert than the wake group (F(1, 158) =
26.55, p<.001, η
p2
= 0.14). This pattern of results is however contrary to what would be
expected if alertness levels were driving the retention advantage for the sleep (vs wake) groups,
and therefore cannot explain the purported benefit of sleep for memory. There was no main
effect of Expectancy and no Delay�Expectancy interaction at the 12-hout test (all p>.05).
Performance in the PVT (Table 2) was comparable across the Delay and Expectancy condi-
tions at the baseline and 12-hour tests (all p>.05). At the 7-day test, we observed a
Delay�Expectancy interaction (F(1,158) = 7.69, p= .006, η
p2
= 0.05). Post-hoc comparisons
show that this effect was driven by a greater number of attentional lapses in the wake-expected
condition compared to the sleep-expected condition (p
bonf
= .015). No other post-hoc compar-
isons were significant (p
bonf
>.05). There were no main effects of Delay or Expectancy at the
7-day test (all p>.05).
Experiment 2
In Experiment 1, we failed to replicate the finding that sleep selectively strengthens memories
that are deemed relevant for the future [24]. One potential explanation for this failed replica-
tion is that baseline performance levels were too high to allow knowledge of a future test to
Fig 2. Experiment 1 Memory retention. Performance at the 12-hour (A) and 7-day (B) tests in Experiment 1.
Performance is indicated by the percentage of word pairs recalled at each test, with performance on the final baseline
test set to 100%. Data are shown as mean ±SEM; �p<.05, ��p<.01.
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Table 1. Stanford Sleepiness Scale (SSS) scores for each condition and each session of Experiment 1.
Baseline 12-hour test 7-day test
Expected Unexpected Expected Unexpected Expected Unexpected
Sleep 2.64 ±0.14 3.00 ±0.11 3.24 ±0.24 3.51 ±0.23 3.02 ±0.22 3.44 ±0.25
Wake 2.94 ±0.14 2.94 ±0.25 2.24 ±0.11 2.45 ±0.22 2.90 ±0.11 3.07 ±0.20
Data are presented as means ±SEM.
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augment sleep-associated consolidation. Baseline memory performance in our first experi-
ment was 77%, although true performance levels were likely far higher due to the feedback
provided at the final baseline testing round. Indeed, accuracy levels at the 12-hour test
exceeded baseline performance despite there being no further exposure to the word pairs. To
address the possibility that memories were encoded too strongly to permit a selective benefit
of sleep on future-relevant information, we ran a second, pre-registered experiment (osf.io/
3k6ej) in which baseline performance levels were reduced relative to Experiment 1.
Methods
Participants. Two-hundred and thirteen adults (126 women, mean ±SD age: 24.45 ±3.86
years) were recruited using the same procedures as described in Experiment 1. As before, only
participants who expected/did not expect a memory test in line with their respective condi-
tions were included in our analysis (a total of 82 participants were excluded on this basis,
sleep-expected n= 5, sleep-unexpected n= 32, wake-expected n= 8, wake-unexpected n= 37).
A further seven participants were excluded from the analysis for either taking a daytime nap
during the ‘wake’ delay of the experiment (n= 6) or reporting to have obtained no sleep during
the ‘sleep’ delay (n= 1). This resulted in a final sample of 124 participants (78 women,
mean ±SD age: 24.32 ±4.05 years) who were allocated equally across conditions (n= 31 in
each condition).
Procedure. There were three key modifications to the procedures: 1) the number of sti-
muli was increased from 40 to 100 word pairs, 2) the criterion participants were required to
meet during the baseline assessment was reduced from 60% to 40% correct responses, and 3)
only a 12-hour follow-up test was included (because no changes were observed between the
12-hour and 7-day tests in Experiment 1). Sixty-eight percent of participants met the baseline
performance criterion on the first round of testing, whereas the remaining participants met
this criterion on the second testing round. The proportion of participants who completed a
second test did not differ between the sleep and wake groups, X
2
(1, N= 124) = 1.83, p= .176.
Results
Baseline memory performance
A 2 (Delay: Sleep/Wake) �2 (Expectancy: Expected/Unexpected) �2 (Experiment: 1/2)
ANOVA confirmed that baseline memory performance was significantly lower in Experiment
2 (64% ±1.29; ±SEM) relative to Experiment 1 (77%; F(1, 278) = 65.08, p<.001, η
p2
= 0.19,
Table 3). All other main effects and interactions were not significant (p>.05), and this was
also the case when baseline performance in Experiment 2 was assessed in isolation (Delay: F(1,
120) = 0.01, p= .917; Expectancy: F(1, 120) = 0.54, p= .464); Delay�Expectancy: (F(1, 120) =
0.005, p= .946).
Table 2. Percentage of PVT trials with attentional lapses (RT >500 ms) in Experiment 1.
Baseline 12-hour test 7-day test
Expected Unexpected Expected Unexpected Expected Unexpected
Sleep 7.19 ±1.32 9.81 ±2.32 11.64 ±2.05 11.57 ±1.65 10.40 ±2.26 14.82 ±1.76
Wake 10.03 ±2.08 8.86 ±1.97 13.24 ±2.63 9.52 ±2.30 20.38 ±2.90 11.04 ±2.14
Data are presented as means ±SEM.
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Memory retention
Our findings mirrored those of Experiment 1. Memory retention at the 12-hour test was
higher after sleep than wakefulness (F(1, 120) = 5.27, p= .023, η
p2
= 0.04, Fig 3). However, per-
formance was not influenced by knowledge of a future test (F(1, 120) = 0.17, p= .682) and
there was no Delay�Expectancy interaction (F(1, 120) = 0.45, p= .504). A Bayesian ANOVA
yielded a Delay�Expectancy BF
01
of 3.42, indicating moderate evidence in favour of the null
hypothesis. As before, our findings suggest that sleep-associated memory benefits were not
amplified for individuals who expected a test.
Self-reported sleep
Mean (±SEM) hours slept was comparable between the sleep-expected (7.05 ±0.25) and the
sleep-unexpected (6.69 ±0.29) conditions (t(60) = 0.93, p= .359). Again, to ensure that our
results were not influenced by participants who reported less than 6 h of sleep, we repeated our
analyses without these participants (n= 9). Our findings were replicated, with only a
Table 3. Baseline memory performance for each condition in Experiment 1 and Experiment 2. Performance was calculated as the percentage of correctly recalled
word pairs at the final baseline testing round.
Experiment 1 Experiment 2
Expected Unexpected Expected Unexpected
Sleep 77.56 ±1.58 77.12 ±1.96 62.74 ±2.37 64.84 ±2.64
Wake 75.85 ±1.94 75.92 ±2.14 62.65 ±2.65 64.39 ±2.76
Data are presented as means ±SEM.
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Fig 3. Experiment 2 Memory retention. Perfomance at the 12-hour test in Experiment 2. Performance is indicated by
the percentage of word pairs recalled at test, with performance on the final baseline test set to 100%. Data are shown as
means ±SEM; �p<.05.
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significant memory benefit of sleep emerging from the ANOVA (F(1, 111) = 6.39, p= .013, η
p2
= 0.05). There were no other significant main effects or interactions (all p>.05).
As before, there was no significant correlation between self-reported sleep and task perfor-
mance in either the sleep-expected (r= 0.09, p= .633) or sleep unexpected condition (r= 0.11,
p= .558).
Alertness
SSS scores (Table 4) were comparable across conditions at the baseline test (all p>.05), how-
ever as observed in Experiment 1, at the 12-hour test participants in the sleep groups rated
themselves as less alert than those in the wake groups (F(1,120) = 5.12, p= .026, η
p2
= 0.04).
There was no main effect of Expectancy and no Delay�Expectancy interaction in this analysis
(all p>.05).
The number of attentional lapses made by participants in the PVT (Table 5) were equivalent
across Delay and Expectancy conditions at both the baseline test and the 12-hour test, no main
effects or interaction effects were observed at either session (all p>.05).
Discussion
In the current study, we sought to replicate the seminal finding that sleep preferentially
strengthens memories that are deemed to be relevant for the future [24]. In Experiment 1, we
ran an online replication of Wilhelm et al. [24], using a verbal paired associates paradigm. We
observed a benefit of sleep for the retention of word pairs, but, unlike the original study, we
found no evidence that sleep-associated memory effects were influenced by knowledge of a
future test. In Experiment 2, we reduced pre-sleep memory performance to account for the
potential impact of memory strength on overnight consolidation [17,36,37,44–46]. We repli-
cated the results of Experiment 1, observing an overall memory benefit of sleep, but not a selec-
tive influence of sleep on future-relevant information.
To our knowledge, this is the first study to attempt a direct replication of Wilhelm et al. [24]
using the same paired associates protocol. Although our results do not support the original
findings, they are in keeping with a growing number of studies that have also failed to observe
an amplified benefit of sleep on future-relevant information [34,35]. The finding that sleep
does not offer a selective memory benefit for future relevant information has therefore been
Table 4. Stanford Sleepiness Scale (SSS) scores for each condition and each session of Experiment 2.
Encoding 12-hour test
Expected Unexpected Expected Unexpected
Sleep 2.58 ±0.13 2.58 ±0.12 3.48 ±0.29 3.26 ±0.23
Wake 2.77 ±0.19 2.84 ±0.14 2.84 ±0.21 2.81 ±0.23
Data are presented as means ±SEM.
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Table 5. Percentage of PVT trials with attentional lapses (RT >500ms) in Experiment 2.
Encoding 12-hour test
Expected Unexpected Expected Unexpected
Sleep 9.81 ±2.22 8.00 ±2.08 22.26 ±3.66 14.51 ±3.09
Wake 11.36 ±2.41 10.26 ±2.06 15.55 ±3.53 11.72 ±2.30
Data are presented as means ±SEM.
https://doi.org/10.1371/journal.pone.0258110.t005
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Sleep and selective memory consolidation
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shown across a number of different declarative and non-declarative memory paradigms.
These findings also complement a broader literature, where selective sleep-associated memory
effects are not always replicated. For example, emotionally salient information is widely con-
sidered to be particularly sensitive to consolidation during sleep [47], but many studies have
failed to find a preferential impact of sleep on emotionally negative relative to neutral memo-
ries [6,26–29,48]. To what extent, and under what conditions, sleep offers selective benefits to
memory is therefore still largely unknown.
Previous studies suggest that the memory benefits of sleep are amplified for weakly-
encoded relative to strongly-encoded information [17,36,37,45,46,49,50]. We therefore rea-
soned that baseline performance levels in Experiment 1 may have been too high to allow
knowledge of a future test to augment sleep-associated consolidation. To address this issue, we
modified our task in a second experiment to reduce performance at baseline. Mirroring Exper-
iment 1, memory retention was superior after sleep than wakefulness, but this effect was com-
parable for individuals who did and did not expect a future test. Hence, memory strength at
encoding does not appear to explain our failure to replicate the findings of Wilhelm et al. [24].
It is nevertheless important to emphasise that our study was carried out in an online envi-
ronment and not in a laboratory. The experimental control afforded by a laboratory may have
had a major impact on how sleep influenced memory consolidation in the original investiga-
tion by Wilhelm et al. [24]. Of particular importance was the physical presence of an experi-
menter, which would have presumably increased the salience of instructions pertaining to
future tests. Other factors that can be more readily measured in the laboratory include the
time between encoding and bed time, overall sleep time and sleep architecture. Although
online questionnaires can provide some insight into participant sleep practices, they do not
offer the same degree of precision as objective laboratory measures such as polysomnography.
Our findings therefore demonstrate that a selective overnight strengthening of future-relevant
information does not emerge in an online environment, although further work is necessary to
determine whether such effects replicate in the laboratory.
Beyond the experimental environment, there were also several minor procedural differ-
ences between this study and Wilhelm et al. [24], which might have influenced our findings.
Notably, we did not attempt to control the time between completion of the experimental tasks
and sleep onset. There is evidence to suggest that sleep-memory effects are amplified when
sleep closely follows learning [17,51,52], and a shorter delay between learning and sleep might
also facilitate the selective strengthening of future-relevant information. Further procedural
differences between our study and Wilhelm et al. include our use of an explicit encoding
instruction (i.e. we asked participants to imagine the words of each pair interacting whereas
Wilhelm et al did not report doing so), and our puzzle matching (interference) task, which was
shorter in duration than the video game used by Wilhelm et al. It is possible that these proce-
dural differences reduced the sensitivity of our protocol to selective memory strengthening in
sleep.
Our results are however, in keeping with an emerging body of online research investigating
sleep-associated consolidation. For example, Kroneisen & Kuepper-Tetzel [51] also observed a
benefit of overnight sleep (vs daytime wakefulness) on the retention of verbal paired associates
using a web-based task. To our knowledge, we are the first to show in an online study that the
memory effects of sleep are retained across a longer retention interval of 7 days. This effect is
in line with numerous laboratory studies that have shown long-term benefits of sleep for mem-
ory [53–55] and supports the view that sleep promotes the retention of durable and long-last-
ing memory representations [1–3].
A general concern in studies comparing memory retention after overnight sleep and day-
time wakefulness is that participants may be more alert in the morning after sleep, as
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Sleep and selective memory consolidation
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compared to the evening after wakefulness, meaning that an apparent memory benefit of sleep
could be driven by between-condition differences in alertness. However, in both experiments
of the current study, the sleep groups rated themselves as less alert than the wake groups when
they returned to complete the test phase. This is precisely the opposite pattern of results that
would be expected if between-group differences in alertness could explain the purported mem-
ory effect of sleep. We cannot rule out an impact of other circadian factors pertaining to even-
ing and morning testing, particularly given that participants were allocated to their respective
conditions based on the time of day that they signed up for the study (although they were
unaware of the crucial sleep vs wake manipulation). Many other studies have however reported
superior retention after a daytime nap relative to a corresponding interval of wakefulness, sug-
gesting that sleep-associated memory benefits are not driven by time-of-day effects.
In sum, we were unable to replicate the seminal finding that sleep selectively strengthens
memories that are deemed relevant for the future [24]. Our findings, derived from two online
experiments, are in keeping with a number of recent studies that have also failed to observe a
targeted effect of sleep on future-relevant information [34,35]. However, given the important
differences in experimental control between our online study and the original laboratory
experiment of Wilhelm et al. [24], further work is necessary to determine the conditions under
which these selective sleep-memory effects may emerge. Nevertheless, our findings add to a
developing body of online research indicating that sleep provides robust and long-lasting ben-
efits for memory [56].
Author Contributions
Conceptualization: Jennifer E. Ashton, Scott A. Cairney.
Data curation: Jennifer E. Ashton.
Formal analysis: Jennifer E. Ashton.
Funding acquisition: Scott A. Cairney.
Investigation: Jennifer E. Ashton.
Methodology: Jennifer E. Ashton, Scott A. Cairney.
Project administration: Jennifer E. Ashton.
Resources: Jennifer E. Ashton, Scott A. Cairney.
Software: Jennifer E. Ashton.
Supervision: Scott A. Cairney.
Validation: Jennifer E. Ashton.
Visualization: Jennifer E. Ashton, Scott A. Cairney.
Writing – original draft: Jennifer E. Ashton, Scott A. Cairney.
Writing – review & editing: Jennifer E. Ashton, Scott A. Cairney.
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