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RESEARCH ARTICLE
Sleep bolsters schematically incongruent
memories
Jennifer E. Ashton
1
, Bernhard P. Staresina
2
, Scott A. CairneyID
1,3
*
1Department of Psychology, University of York, York, United Kingdom, 2Department of Experimental
Psychology, University of Oxford, Oxford, United Kingdom, 3York Biomedical Research Institute, University
of York, York, United Kingdom
*scott.cairney@york.ac.uk
Abstract
Our ability to recall memories is improved when sleep follows learning, suggesting that
sleep facilitates memory consolidation. A number of factors are thought to influence the
impact of sleep on newly learned information, such as whether or not we rehearse that infor-
mation (e.g. via restudy or retrieval practice), or the extent to which the information is consis-
tent with our pre-existing schematic knowledge. In this pre-registered, online study, we
examined the effects of both rehearsal and schematic congruency on overnight consolida-
tion. Participants learned noun-colour pairings (e.g. elephant-red) and rated each pairing as
plausible or implausible before completing a baseline memory assessment. Afterwards, par-
ticipants engaged in a period of restudy or retrieval practice for the pairings, and then
entered a 12 h retention interval of overnight sleep or daytime wakefulness. Follow-up
assessments were completed immediately after sleep or wake, and again 24 h after learn-
ing. Our data indicated that overnight consolidation was amplified for restudied relative to
retested noun-colour pairings, but only when sleep occurred soon after learning. Further-
more, whereas plausible (i.e. schematically congruent) pairings were generally better
remembered than implausible (i.e. schematically incongruent) pairings, the benefits of sleep
were stronger for implausible relative to plausible memories. These findings challenge the
notion that schema-conformant memories are preferentially strengthened during post-learn-
ing sleep.
Introduction
Memories fade over time, but rehearsing learned materials can help to improve recall. There
are two rehearsal strategies that an individual can use to help commit new information to
memory: restudy and retrieval practice. For example, one might try to revise for a spelling test
by re-reading the words (restudy) or by attempting to recall the words from memory (retrieval
practice). In the short term (e.g. within an hour of learning), restudied information is better
remembered than information subjected to retrieval practice [1–3]. In the longer-term (e.g.
from several hours up to a week after learning), by contrast, the benefits of retrieval practice
tend to outweigh those arising from restudy [3,4]. This difference in retention after longer
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OPEN ACCESS
Citation: Ashton JE, Staresina BP, Cairney SA
(2022) Sleep bolsters schematically incongruent
memories. PLoS ONE 17(6): e0269439. https://doi.
org/10.1371/journal.pone.0269439
Editor: Maria Wimber, University of Glasgow,
UNITED KINGDOM
Received: November 5, 2021
Accepted: May 21, 2022
Published: June 24, 2022
Peer Review History: PLOS recognizes the
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https://doi.org/10.1371/journal.pone.0269439
Copyright: ©2022 Ashton et al. 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 can be
retrieved via the following link: osf.io/vcfgm.
Funding: This work was supported by a Medical
Research Council (https://mrc.ukri.org/) Career
Development Award (MR/P020208/1) to S.A.C.
The funders had no role in study design, data
intervals suggests that restudied memories might be more susceptible to decay than those sub-
jected to retrieval practice.
Memory retention is improved by post-learning sleep [5–11], suggesting that sleep supports
the consolidation of newly learned information. Contemporary models of sleep-associated
memory processing propose that memories are reactivated during sleep, and thereby inte-
grated into long-term storage [5,12–16]. However, sleep does not benefit all memories equally,
with accumulating evidence suggesting that overnight memory gains are more robust for
weakly encoded than strongly encoded materials [17–19, but also see 20]. Relatedly, efforts to
enhance overnight consolidation via memory cueing in sleep are most effective when pre-sleep
learning performance is low [21–23].
Given that sleep may provide the greatest benefit to weakly encoded memories, and that
restudied memories are more prone to decay than memories subjected to retrieval practice,
restudied information should be particularly responsive to overnight consolidation. Consistent
with this view, previous work has indicated that restudied but not retested materials are better
remembered after a night of sleep than a day of wakefulness [24,25]. It has thus been suggested
that retrieval practice may prompt a rapid consolidation of newly learned information into
long-term memory, potentially via similar mechanisms to those underpinning sleep-associated
memory processing [26].
Information that is congruent with pre-existing schematic knowledge is typically better
remembered than schematically incongruent information [27–29]. The memory benefits of
cognitive schemata are thought to arise from interactions between prefrontal cortex and
medial temporal lobe, which support efficient learning of schematically congruent materials
[30,31]. Interestingly, prior knowledge also enhances sleep-associated consolidation, such that
schema-conformant memories are strengthened during sleep to a greater extent than non-con-
formant memories [32–34]. The interleaved reactivation of new memories and their associated
schematic representations during sleep is thought to facilitate the integration of newly learned
information into long-term storage [12].
In this pre-registered, online study (osf.io/f82mw), we tested two hypotheses: 1) the benefits
of sleep (vs wakefulness) for memory will emerge for restudied memories but not memories
subjected to retrieval practice, and 2) the retention advantage for schematically congruent (vs
incongruent) memories will be stronger after sleep than wakefulness. We also tested a third
novel hypothesis concerning the combined effects of sleep, memory rehearsal and prior
knowledge on memory consolidation: assuming that overnight memory processing preferen-
tially strengthens restudied (vs retested) and schematically congruent (vs incongruent) memo-
ries, then, after sleep (vs wake), the benefits of restudy (vs retrieval practice) should be greater
for schema-conformant than non-conformant information.
We tested our three hypotheses using a source memory paradigm, in which participants
learned noun-colour pairings (e.g. elephant-red) and rated the plausibility of each pairing (e.g.
a red elephant is implausible). From this plausibility response, we could infer the schematic
congruency of each pairing, which was based on each participant’s unique understanding of
the world. Training (encoding and baseline memory assessment) took place in the morning or
evening and was immediately followed by a memory rehearsal phase, during which partici-
pants engaged in a period of restudy or retrieval practice for the pairings. Memory for the pair-
ings was re-assessed 12 h later, following a night of sleep (evening training) or a day of
wakefulness (morning training). This allowed us to determine the effects of sleep (vs wake) on
the consolidation of memories that had been restudied or retested, and were plausible (i.e.
schematically congruent) or implausible (i.e. schematically incongruent). Finally, so that we
could determine the effects of sleep on memory retention after a longer delay, participants also
completed a second follow-up assessment 24 h after training.
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collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Methods
Participants
One-hundred and sixty-eight adults were recruited via the online platform Prolific (app.pro-
lific.co/). Participants were aged between 18 and 30 years and reported to be living in the UK
with English as their first language. On the days of the study, participants were asked to follow
their usual daily routines, abstain from alcohol and avoid taking naps (as is standard practice
in our lab e.g., [35–38]. Informed consent was obtained from all participants in line with the
Research Ethics Committee of the Department of Psychology at the University of York, who
approved the study. Participants were required to meet a memory performance criterion in
the first session (see below) to continue with the remaining sessions. A total of 102 participants
met this criterion and were invited to take part in sessions two and three. Sixty-two partici-
pants returned to complete the full study. We analysed data from a final sample of 60 partici-
pants after excluding two participants who reported napping during the days of the
experiment. The remaining participants took part in a sleep (n= 30, 18 females, mean ±SD
age = 25.07 ±4.02 years) or wake group (n= 30, 25 females, mean ±SD age = 25.00 ±3.65
years).
Our sample size was calculated using an effect size reported in Antony & Paller (2018) [25].
The effect of interest was an interaction (η
p2
= 0.14) from a two-way ANOVA with the factors
Delay (AM/PM) and rehearsal strategy (Restudy/Retest). We determined that a minimum
sample size of N= 20 would be necessary for 95% power to detect an effect of this magnitude.
However, because Antony & Paller (2018) [25] conducted their study in the laboratory, we
believed it reasonable to increase our sample to N= 60 to mitigate any noise associated with
online data collection. Data collection continued until our desired sample size was met, with
each participant providing a full and useable data set.
Materials and software
Six-hundred and eighty English nouns referring to concrete objects were obtained from the
Medical Research Council Psycholinguistics database (websites.psychology.uwa.edu.au/
school/mrcdatabase/uwa_mrc.htm). Words were three to eight letters long, with a Kucera–
Francis written frequency of 10–100. Only words with concreteness and imageability ratings
ranging from 400 to 700 (out of 700) were included [39]. For each participant, 240 nouns were
randomly selected for the encoding phase and were each paired with one of four colours (red,
yellow, green or blue). An equal number of nouns were paired with each colour (i.e. 60 nouns
per colour). The remaining nouns were randomly assigned to each of the three test phases as
foils (120 in each test). The experimental tasks were programmed using PsychoPy and hosted
on Pavlovia.org [40]. Participants completed the tasks at home on a desktop or laptop (the use
of tablets or smartphones was prohibited).
Procedure overview
The study was divided into three sessions (see Fig 1A). Session one began at 8am (wake group)
or 8pm (sleep group), and comprised an encoding phase, a baseline memory test and a mem-
ory rehearsal phase. Follow-up memory tests were completed at sessions two and three, which
began 12 h and 24 h after session one, respectively. The Stanford Sleepiness Scale [41] and a
three-minute psychomotor vigilance task [42,43] were completed at each session to measure
participant alertness.
Encoding. There were 240 encoding trials. Each trial began with a fixation cross, pre-
sented in the centre of the screen for 500 ms. A randomly selected noun was then displayed
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Fig 1. Experimental procedure and tasks. (A) Participants completed an encoding phase, a baseline memory test and a memory rehearsal phase in the
morning (wake group) or evening (sleep group). Participants returned 12 h and 24 h later to complete follow-up memory tests. (B) On each encoding trial,
participants were presented with a noun and a coloured square. Participants were asked to imagine the referent of the noun in the given colour and indicate
whether it was a plausible or implausible combination. (C) On each test trial, participants were asked to decide whether a noun was ‘old’ (i.e. they had seen the
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above a coloured square for 3 s (see Fig 1B). Participants were asked to imagine the referent of
the noun in the given colour (red, yellow, green or blue) and indicate whether it was a plausible
or implausible combination (e.g. a red elephant is implausible). Any trial for which a partici-
pant failed to provide a plausible or implausible response was removed from all subsequent
analyses. A mean of 3.11% (SEM ±0.64) trials were removed on this basis. Across both the
sleep and wake groups, participants rated 57% (SEM ±1.41) of the noun-colour pairings as
plausible.
Baseline test. There were 360 test trials, 240 of which corresponded to the nouns pre-
sented at encoding. The remaining 120 trials were unseen foils. On each trial, a central fixation
cross (500 ms) preceded a randomly selected noun (see Fig 1C). Participants were asked to
indicate whether the noun was ‘old’ (i.e. they recognised it from encoding) or ‘new’ (i.e. they
did not recognise it from encoding). For each ‘old’ response, participants were also asked to
select the colour that had been paired with the noun at encoding, and guess if they were not
certain. For each ‘new’ response, participants were asked to indicate the location of a letter ‘X’
that was randomly presented in one of four locations. Employing this two-step approach pro-
vided an index of item memory (noun recognition) and source memory (colour recall). Note
that we refer to colour recall as source memory because the colour associated with each noun
reflects the context or source with which that noun was encountered at learning.
To motivate engagement with the task, participants were provided with an overall perfor-
mance score at the end of the baseline test (calculated as the percentage of correct old/new
responses) and were encouraged to try and improve on their score in the next test.
Performance criteria. If a participant did not meet the following baseline performance
criteria, they were not invited back for the follow-up tests and a new participant was recruited
in their place. We excluded 66 participants on this basis.
The performance criteria were set in two steps:
1. Item memory score of 50% or above. Calculated as the proportion of correctly recognised
‘old’ nouns, minus the proportion of foils incorrectly rated as ‘old’.
2. Source memory score of 40% or above. Calculated as the proportion of correctly identified
colours (restricted to correctly recognised nouns), referred to hereafter as colour hits.
Memory rehearsal. After the baseline test, participants completed two rehearsal phases:
restudy and retrieval practice (order counterbalanced across participants). Noun-colour pair-
ings that were correctly retrieved at baseline were randomly allocated to three conditions:
restudy, retrieval practice and a no-rehearsal control condition. The restudy and retrieval prac-
tice phases followed the same procedures as at encoding and the baseline test, respectively. Par-
ticipants received only one additional exposure to each noun-colour pairing allocated to the
restudy and retrieval practice conditions. Pairings in the no-rehearsal condition were not
revisited until the follow-up tests.
To check whether there was any bias in the distribution of noun-colour pairings between
conditions, we applied colour hits at the baseline test to a 2 (Plausibility: Plausible/Implausible)
3 (Rehearsal Strategy: Restudy/Retrieval Practice/No-Rehearsal) 2 (Group: Sleep/Wake)
mixed ANOVA. There was a main effect of Plausibility (F(1, 58) = 21.73, p<.001, η
p2
= 0.27),
reflecting participants’ tendency to rate more noun-colour pairings as plausible than
noun at encoding) or ‘new’ (i.e. they had not seen the noun at encoding). For ‘old’ responses, participants were asked to select the colour that had appeared
with the noun at encoding. For ‘new’ responses participants were asked to indicate the location of the letter ‘X’.
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implausible at encoding. There was also a main effect of Rehearsal Strategy (F(2,116) = 3.13, p
= .048, η
p2
= 0.05), reflecting a slight difference in the number of pairings allocated to each
rehearsal condition (mean ±SEM, Restudy: 21.08 ±0.64, Retrieval Practice: 20.82 ±0.65, No-
Rehearsal: 20.83 ±0.66). There was no main effect of Group and none of the interactions were
significant (p>.05).
The main effect of Rehearsal Strategy emerged because of a randomisation error, where on
occasions that the number of baseline colour hits was not divisible by 3, the first ‘additional’
noun-colour pairing was always allocated to the Restudy condition. For example, if 61 noun-
colour pairings were correctly recalled at baseline, then 20 would be allocated to Retrieval
Practice, 20 would be allocated to No-Rehearsal and the remaining 21 would be allocated to
Restudy.
Follow-up tests. Participants completed a follow-up test 12 h after the baseline test (i.e.
after a night of sleep [sleep group] or a day of wakefulness [wake group]). A second follow-up
test took place 24 h after the baseline assessment. Both follow-up tests followed the same proce-
dures as the baseline test, with the exception that a new set of foils were used in each. Partici-
pants were not asked if they had undertaken any active rehearsal of the noun-colour pairings
during the intervals between experimental sessions.
Statistical analysis
Item memory. D-prime [normalised (hits / hits + misses)–normalised (false alarms / false
alarms + correct rejections)] was calculated to assess recognition accuracy for nouns (item
memory) at the baseline test and the 12/24 h follow-ups. Independent samples t-tests compar-
ing d-prime between the sleep and wake groups were performed at each test.
Source memory. Our main dependent variable of interest was source memory retention,
calculated separately for the 12 h and 24 h follow-ups as the number of colour hits divided by
the number of colour hits at baseline (converted to percentages). Data at each follow-up was
applied to a 2 (Group: Sleep/Wake) 3 (Rehearsal Strategy: Restudy/Retrieval Practice/No-
Rehearsal) 2 (Plausibility: Plausible/Implausible) mixed ANOVA. The plausibility of each
noun-colour pairing was determined by the plausible/implausible responses provided at
encoding. Significant interactions were interrogated using post-hoc comparisons with a Bon-
ferroni-corrected alpha adjustment for the number of tests performed. All statistical analyses
were performed in R [44].
Results
Item memory
There was no difference in item memory (noun recognition accuracy, d-prime) between the
sleep and wake groups at the baseline test (sleep group mean ±SEM: 2.09 ±0.07, wake group:
2.11 ±0.10, t(58) = 0.21, p= .839). However, at the 12 h follow-up, item memory was higher
for participants who had slept (2.00 ±0.10) relative to those who had remained awake
(1.71 ±0.11, t(58) = 2.01, p= .049, d= 0.52). No between-group difference was present at the
24 h follow-up (sleep group: 2.01 ±0.10, wake group: 1.97 ±0.13, t(58) = 0.26, p= 0.80).
Table 1. Source memory scores (colour hits) at the baseline test for the sleep and wake groups. Scores are pre-
sented separately for each plausibility condition. Data are presented as mean ±SEM.
Plausible Implausible
Sleep 72.99 ±2.26 60.85 ±2.13
Wake 71.79 ±2.23 59.10 ±2.55
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Source memory: Baseline
Source memory scores (colour hits) at baseline (see Table 1) were applied to a 2 (Group: Sleep/
Wake) 2 (Plausibility: Plausible/Implausible) mixed ANOVA. There was a main effect of
Plausibility (F(1, 58) = 127.22, p<.001, η
p2
= 0.69), as plausible noun-colour pairings were
better remembered than implausible pairings. However, there was no main effect of Group (F
(1, 58) = 0.23, p= .630) and no interaction between factors (F(1, 58) = 0.06, p= .802).
Source memory: Sleep and rehearsal
We first tested the hypothesis that a memory benefit of sleep would emerge for restudied
source memories but not source memories subjected to retrieval practice.
12 h follow-up. There was a main effect of Group (F(1, 58) = 11.39, p= .001, η
p2
= 0.16),
indicating that source memory retention was significantly higher after a night of sleep than a
day of wakefulness (see Table 2 and Fig 2). There was also a main effect of Rehearsal Strategy
(F(2, 116) = 76.53, p<.001, η
p2
= 0.57): retention scores were higher in 1) the restudy vs no-
rehearsal condition (t(59) = 12.58, p<.001, d= 1.62), 2) the retrieval practice vs no-rehearsal
condition (t(59) = 8.51, p<.001, d= 1.10), and 3) the restudy vs retrieval practice condition (t
(58) = 2.90, p= .005, d= 0.38), indicating that restudy produced the greatest overall retention
benefit. However, there was no Group Rehearsal Strategy interaction (F(2, 116) = 1.70, p=
.187), indicating that the memory effects of sleep (vs wake) were not amplified in any of the
rehearsal conditions.
24 h follow-up. There was no main effect of Group at the 24 h follow-up (F(1, 58) = 0.39,
p= .535), but the main effect of Rehearsal Strategy remained (F(2, 116) = 83.31, p<.001, η
p2
=
0.59): retention scores were again higher in 1) the restudy vs no-rehearsal condition (t(58) =
13.72, p<.001, d= 1.77), 2) the retrieval practice vs no-rehearsal condition (t(58) = 8.81, p<
.001, d= 1.14), and 3) the restudy vs retrieval practice condition (t(58) = 2.67, p= .010,
d= 0.35). Interestingly, a significant Group Rehearsal Strategy interaction also emerged at
the 24 h follow-up (F(2, 116) = 3.63, p= .029, η
p2
= 0.06, see Fig 3). Whereas source memory
retention was significantly higher after restudy than retrieval practice in the sleep group (t(29)
= 4.11, p<.001, d= .75), no such difference emerged in the wake group (t(29) = 0.49, p=
.626). Hence, after a 24 h delay, the retention advantage for restudied (vs retested) information
was only evident among individuals who had slept soon after learning.
Subsidiary analyses. Although the foregoing analyses of source memory retention were
restricted to colours for which the associated noun was correctly recognised at baseline, they
did not control for noun recognition (item memory) at the 12 h or 24 h follow-ups. This is an
important consideration, because if a participant failed to re-recognise a noun at the 12 h fol-
low-up, the trial was scored as a source memory error, meaning that any effect of sleep on item
Table 2. Source memory scores (%) for the 12 h and 24 h follow-ups in the sleep and wake groups. Scores are presented separately for each rehearsal strategy and plau-
sibility condition. Data are presented as mean ±SEM.
Restudy Retrieval Practice No-Rehearsal
Plausible Implausible Plausible Implausible Plausible Implausible
12 h Follow-up
Sleep 86.99 ±2.01 81.35 ±3.12 82.26 ±2.11 73.05 ±3.15 73.28 ±2.48 60.55 ±2.46
Wake 77.49 ±3.04 66.40 ±4.34 75.11 ±2.51 64.05 ±3.52 62.93 ±2.78 44.29 ±3.38
24 h Follow-up
Sleep 80.25 ±3.00 74.56 ±3.72 76.37 ±3.01 64.66 ±3.71 67.37 ±3.67 54.38 ±3.32
Wake 79.41 ±2.83 67.80 ±3.78 76.74 ±2.60 68.07 ±2.67 63.46 ±2.84 47.84 ±3.43
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memory could have been misinterpreted as an effect of sleep on source memory. To address
this concern, we repeated the above analyses (and those described in the next section), but
fully controlled for item memory by restricting the input data to colours for which the associ-
ated noun was correctly recognised at both the baseline test and the relevant 12 h or 24 h fol-
low-up (see S1 File). In brief, these subsidiary analyses confirmed the results of our main
analyses, although the Group Rehearsal Strategy interaction at the 24 h follow-up became
non-significant (F(2,116) = 2.47, p= .089, η
p2
= 0.04).
Source memory: Sleep and schematic congruency
Next, we tested the hypothesis that overnight consolidation strengthens schematically congru-
ent (plausible) source memories to a greater extent than schematically incongruent (implausi-
ble) source memories.
12 h follow-up. There was a main effect of Plausibility (F(1, 58) = 109.64, p<.001, η
p2
=
0.65), indicating that source memory retention was higher for plausible relative to implausible
noun-colour pairings (see Fig 4A). Importantly, there was also a significant Group Plausibil-
ity interaction (F(1, 58) = 4.10, p= .048, η
p2
= 0.07), but not in the direction predicted. Post-
Fig 2. Source memory retention and rehearsal, 12h follow-up. Source memory retention at the 12 h follow-up for the sleep and wake groups. Data is shown
for each rehearsal condition, collapsed across the plausibility conditions. Error bars represent SEM. p<.001.
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hoc comparisons revealed that the retention advantage for plausible (vs implausible) pairings
was smaller in the sleep group (t(29) = 6.18, p<.001, d= 1.13) than the wake group (t(29) =
8.56, p<.001, d= 1.56, see Fig 4A). Moreover, the memory benefits of sleep (vs wakefulness)
were stronger for implausible pairings (t(58) = 3.41, p= .001, d= .88) than plausible pairings (t
(58) = 3.00, p= .004, d= .78). Taken together, these findings suggest that sleep preferentially
facilitates the consolidation of implausible (and schematically incongruent) associations.
24 h follow-up. The main effect of Plausibility was still present at the 24 h follow-up (F(1,
58) = 87.36, p<.001, η
p2
= 0.60), but the Group Plausibility interaction no longer remained
(F(1, 58) = 0.61, p= .439, see Fig 4B). However, under the assumption that sleep preferentially
strengthens implausible associations, this null effect is to be expected, as participants in the
wake group had also slept before the 24 h follow-up. To test this possibility, we carried out an
exploratory analysis of source memory retention between the 12 h and 24 h follow-ups. If sleep
facilitates the consolidation of schematically incongruent associations, then the retention of
implausible (vs plausible) pairings should be better in the wake group than the sleep group,
because only the wake group have slept between these two time points.
Fig 3. Source memory retention and rehearsal, 24-hour follow-up. Source memory retention at the 24 h follow-up for the sleep and wake groups. Data is
shown for each rehearsal condition, collapsed across the plausibility conditions. Error bars represent SEM. p.001, NS = not significant.
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Source memory retention scores were recalculated as the number of colour hits at the 24 h
follow-up divided by the number of colour hits at the 12 h follow-up (converted to percent-
ages, hence a score >100% indicates an improvement between the two tests) and then reap-
plied to the ANOVA described above. Consistent with the memory benefits of sleep observed
at the 12 h follow-up, a main effect of Group emerged in this new ANOVA, with the wake
group outperforming the sleep group (F(1, 58) = 19.47, p<.001, η
p2
= 0.25).
Importantly, a significant Group Plausibility interaction was also observed (F1, 58) = 8.23,
p= .006, η
p2
= 0.12). Whereas retention was higher for implausible (vs plausible) associations
in the wake group (t(29) = 2.84, p= .008, d= .52), no such difference emerged in the sleep
group (t(29) = 0.92, p= .366, see Fig 5). Taken together with the results of our main analysis,
these findings suggest that overnight consolidation preferentially strengthens schematically
incongruent memories. It is worth noting that the wake group even showed gains in retrieval
performance between the 12 h and 24 h follow-ups (i.e. retention scores >100%), which were
especially pronounced for implausible noun-colour pairings, whereas the sleep group showed
forgetting across both plausible and implausible pairings (thus explaining the non-significant
Group main effect in our main analysis of the 24 h follow-up). There was no main effect of
Rehearsal Strategy (F(2,116) = 0.14, p = 0.87), no Group Rehearsal Strategy interaction (F
(2,116) = 0.69, p = .505) and no Rehearsal Strategy Plausibility interaction (F(2,116) = 1.16, p
= .318) in this exploratory analysis.
Sleep, rehearsal and schematic congruency
Finally, we tested the hypothesis that sleep-associated consolidation is enhanced for schemati-
cally congruent memories that have been restudied prior to sleep. However, there was no sig-
nificant Group Rehearsal Strategy Plausibility interaction at either the 12 h (F(2,116) =
Fig 4. Source memory retention and schematic congruency. Memory retention at the (A) 12 h and (B) 24 h follow-ups for the plausible and implausible
conditions. Data are collapsed across the rehearsal conditions. Error bars represent SEM. p.001, p.01.
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0.31, p= .736) or 24 h follow-up (F(2,116) = 1.37, p= .258). A significant Rehearsal Strategy
Plausibility interaction was observed at the 12 h follow-up F(2, 116) = 0.31, p= .029, η
p2
=
0.06): the retention advantage for plausible (vs implausible) pairings was stronger in the no-
rehearsal condition (t(59) = 7.60, p<.001, d= 0.98) than the retrieval practice (t(59) = 6.30,
p<.001, d= 0.81) or restudy conditions (t(59) = 3.83, p<.001, d= 0.50). This interaction was
not present at the 24 h follow-up (p>.05).
Self-reported sleep
Participants reported the number of hours that they had slept between rehearsal and the 12 h
follow-up (sleep group) or between the 12 h follow-up and the 24 h follow-up (wake group).
Hours slept in the sleep group (mean ±SEM, 6.70 ±0.26) did not significantly correlate with
source memory retention scores in any of the Rehearsal Strategy/Plausibility conditions at
the 12 h follow-up (p>.05). There were also no significant relationships between hours
slept in the wake group (7.37 ±0.18) and source memory retention scores at the 24 h follow-
up (p>.05).
Alertness and vigilance
Stanford sleepiness scale. Scores on the Stanford Sleepiness Scale (see Table 3) were sub-
jected to a 2 (Group: Sleep/Wake) 3 (Session: Encoding/12 h/24 h) mixed ANOVA. There
was a main effect of Session (F(2, 116) = 5.04, p= .008, η
p2
= 0.08), with participants rating
themselves as more alert at encoding than at the 12 h follow-up (t(59) = 3.07, p= .003,
Fig 5. Source memory retention and schematic congruency (recalculated at 24 h). Memory retention at the 24 h
follow-up with retention scores recalculated as the proportion of retention at the 12 h follow-up (scores >100%
indicate an improvement between the time points). Data are collapsed across the rehearsal conditions. Error bars
represent SEM. p.01, NS = not significant.
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d= 0.40). All other between-session differences were non-significant (p>.05). There was no
main effect of Group (F(1, 58) = 2.65, p= .109) and no Group Session interaction (F(2, 116)
= 1.04, p= .358).
Psychomotor vigilance task. Attentional lapses (reaction times >500 ms [45]) on the
psychomotor vigilance task (see Table 3) were subjected to a 2 (Group: Sleep/Wake) 3 (Ses-
sion: Encoding/12 h/24 h) mixed ANOVA. One participant was excluded from this analysis
as no responses were recorded for one of their sessions. There were no main effects of Group
(F(1, 57) = 2.24, p= .140) or Session (F(2, 144) = 0.50, p= .606). A Group Session interaction
was observed (F(2, 114) = 3.75, p= .027, η
p2
= .06), with participants in the sleep group having
more attentional lapses at encoding than participants in the wake group (t(57) = 2.02, p= .048,
d= .53). However, this between-group difference did not survive a Bonferroni-corrected alpha
threshold of p.006.
Discussion
We assessed the impact of rehearsal and prior knowledge on sleep-associated memory consoli-
dation in a pre-registered, online study. In keeping with the previously reported benefits of
sleep for offline memory processing, source memory retention was higher at the 12 h follow-
up in the sleep group than the wake group (an advantage that had subsided by the 24 h follow-
up once participants in the wake group had slept). Although rehearsal strategy had no effect
on the memory benefits of sleep at the 12 h follow-up, a significant interaction emerged at the
24 h follow-up, such that restudied memories were better remembered than retested memo-
ries, but only in the sleep group (i.e. those individuals who had slept soon after learning). How-
ever, this interaction became non-significant in a subsidiary analysis that controlled for item
memory performance at the baseline test and 24 h follow-up. Prior knowledge also influenced
overnight memory processing, but in the opposite direction to that predicted by our hypothe-
sis. The benefits of sleep at the 12 h follow-up were stronger for implausible (i.e. schematically
incongruent) than plausible (i.e. schematically congruent) memories. This selective influence
of sleep on implausible memories was further demonstrated in an exploratory analysis of the
24 h follow-up, where the wake group (having now slept) showed a memory advantage for
implausible relative to plausible information. Our findings therefore demonstrate that over-
night consolidation preferentially strengthens memories that do not conform to pre-existing,
schematic knowledge.
That only the sleep group exhibited greater retrieval of restudied relative to retested memo-
ries at the 24 h follow-up is in keeping with earlier studies assessing the impact of rehearsal
strategy on sleep-associated consolidation [24,25]. It has been suggested that retrieval practice
provides a fast route to consolidation by rapidly strengthening newly acquired memories,
potentially via similar mechanisms to those underpinning the memory benefits of sleep [26].
Overnight memory processes may therefore bypass previously retrieved memories, prioritising
the stabilisation of restudied, and proportionately unconsolidated, information.
Previous studies have demonstrated that overnight consolidation effects are strongest for
information learned within a few hours of sleep [10,46–48]. Our findings build on this prior
Table 3. Scores on the Stanford Sleepiness Scale (SSS) and the proportion of attentional lapses in the psychomotor vigilance task (PVT) for each session. Data are
presented as mean (±SEM).
Encoding 12 h Follow-up 24 h Follow-up
SSS PVT Lapse SSS PVT Lapse SSS PVT Lapse
Sleep 3.07 ±0.19 15.98 ±5.07 3.67 ±0.24 11.94 ±3.48 2.97 ±0.27 9.77 ±2.96
Wake 2.57 ±0.19 5.31 ±1.11 3.10 ±0.24 5.53 ±1.38 2.90 ±0.21 9.49 ±2.93
https://doi.org/10.1371/journal.pone.0269439.t003
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work by suggesting that the selective benefits of sleep for restudied information are contingent
on sleep taking place soon after learning. Future online studies can assess this possibility more
directly by asking participants to report their bed times and determining whether sleep-associ-
ated memory gains for restudied information are amplified among individuals who go to bed
after a short (vs long) post-rehearsal delay. This could have potentially important practical
implications for learning and education, such as the optimal timing of study sessions when stu-
dents are preparing for examinations.
If the benefits of sleep for restudied information are contingent on sleep taking place soon
after learning, then one might have expected to observe a selective effect of sleep on restudied
noun-colour pairings at both the 12 h and 24 h follow-ups. It is worth noting, however, that
although a significant Group Rehearsal Strategy interaction did not emerge at the 12 h fol-
low-up, the pattern of results was very similar to that of the 24 h follow-up (where a significant
Group Rehearsal Strategy interaction did occur), with sleep providing the strongest benefit
for restudied noun-colour pairings (see Figs 2and 3). Interestingly, our exploratory analysis of
the 24 h follow-up (with retention calculated relative to the 12 h follow-up) showed a main
effect of Group, indicating that the wake group (who had slept between the 12 h and 24 h fol-
low-ups) outperformed the sleep group (who had remained awake across the same interval).
Yet, this analysis did not reveal a significant Group Rehearsal Strategy interaction, suggesting
that the memory benefits of delayed sleep are unaffected by prior re-engagement with the
learned materials.
A further point to consider is that participants’ memory for all noun-colour pairings was
assessed at both the 12 h and 24 h follow-ups, which meant that pairings in the restudy condi-
tion had undergone both restudy and retrieval practice by the time of the 24 h assessment. We
therefore cannot rule out the possibility that the selective influence of sleep on restudied infor-
mation at the 24 h follow-up was influenced by retrieval at the 12 h follow-up. We chose to test
all noun-colour pairings at both follow-ups to ensure that there would be a sufficient number
of stimuli for the various rehearsal conditions. For example, despite participants encoding 240
noun-colour pairings, only 125.43 (mean ±SEM, ±3.87) of these were correctly retrieved at
baseline, meaning that each rehearsal condition contained 41.81 (±0.29) pairings. As an alter-
native, we could have assessed retention for half of the noun-colour pairings at the 12 h follow-
up and the other half at the 24 h follow-up, but this would have left only ~21 pairings per
rehearsal condition, which would have been further subdivided by the plausibility condition.
We therefore believe that our chosen approach was optimal, given the various research ques-
tions that we sought to address. Future work focusing on the relationship between overnight
consolidation and rehearsal (independent of schematic congruency) can build on our prelimi-
nary findings by assessing the isolated effects of restudy and retrieval practice on sleep-associ-
ated memory processing.
Our observation that memory retention was improved for plausible (i.e. schematically con-
gruent) relative to implausible (i.e. schematically incongruent) noun-colour pairings is in
keeping with a large body of work on the memory benefits of cognitive schemata [49,50].
However, in direct contrast to our hypothesis, we found that sleep-associated memory gains
were amplified for implausible relative to plausible pairings. We observed this effect in both
the sleep and wake groups after isolating memory retention across their respective intervals of
overnight sleep, suggesting that sleep may offer the greatest advantage to memories that devi-
ate from existing knowledge.
The preferential impact of sleep on schematically incongruent memories appears to oppose
theoretical frameworks arguing that sleep actively underpins both schema formation and the
addition of new knowledge into existing schemata [12]. Because schematically incongruent
declarative associations are thought to be stored as episodic representations in medial temporal
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lobe [31], and newly formed episodic memories are thought to be reactivated in hippocampus
during post-learning sleep [5], it is possible that schematically incongruent episodic memories
are more receptive to overnight consolidation, as compared to schematically congruent infor-
mation. Along these lines, it is worth noting that previous studies reporting enhanced over-
night memory gains for schema-conformant information have used very different memory
paradigms to the current study, which include melodic [32] or spatial navigation tasks [33].
Memory retention at baseline was generally poorer for implausible than plausible noun-col-
our pairings, suggesting that the encoding of implausible pairings was relatively weak. Because
previous work has shown that the memory benefits of sleep are more robust for weakly relative
to strongly encoded materials [17–19, but also see 20], amplified overnight memory gains for
implausible (vs plausible) pairings might reflect an impact of encoding strength, rather than
schematic congruency, on sleep-associated consolidation. Yet, if it were the case that implausi-
ble pairings were selectively strengthened during sleep because they were weakly encoded,
then a three-way interaction should have emerged in our Group Rehearsal Strategy Plausi-
bility ANOVA, with the weakest memories (i.e. non-rehearsed and implausible noun-colour
pairings) showing the greatest enhancement across overnight sleep, but this was not the case.
In conclusion, our online study adds to an extensive body of laboratory-based literature
demonstrating that sleep supports offline memory processing [5–8].Furthermore, our data
suggest that: 1) the memory benefits of sleep are enhanced for restudied information (as com-
pared to information that has undergone retrieval practice), but only when sleep occurs soon
after learning, and 2) sleep-associated memory gains are bolstered for schematically incongru-
ent (relative to schematically congruent) associations. These findings provide new insights
into the interaction of prior knowledge, online rehearsal strategies and offline consolidation
processes.
Supporting information
S1 File. Supplementary results.
(DOCX)
Acknowledgments
Study data can be retrieved via the following link: http://osf.io/vcfgm.
Author Contributions
Conceptualization: Bernhard P. Staresina.
Data curation: Jennifer E. Ashton.
Formal analysis: Jennifer E. Ashton.
Funding acquisition: Scott A. Cairney.
Investigation: Jennifer E. Ashton.
Methodology: Jennifer E. Ashton, Bernhard P. Staresina, Scott A. Cairney.
Project administration: Scott A. Cairney.
Resources: Scott A. Cairney.
Writing – original draft: Jennifer E. Ashton.
Writing – review & editing: Bernhard P. Staresina, Scott A. Cairney.
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