napping sleep memory

Afternoon napping affects memory performance in habitual and non-habitual nappers

Ruth L. F. Leong and her colleagues propose that an afternoon nap improves picture encoding and factual knowledge learning whether one habitually napped or not  (main effects of condition (nap/wake): ps < 0.037). However, they found a significant interaction for the hippocampal-dependent topographical memory task (p = 0.039) wherein a nap, relative to wake, benefitted habitual nappers (HN-nap vs HN-wake: p = 0.003) relative to non-habitual nappers (NN-nap vs. NN-wake: p = 0.918). Notably, for this task, habitual nappers’ performance significantly declined if they were not allowed to nap (HN-wake vs NN-wake: p = 0.037).

A brief summary of mid-day nap literature:

you may obtain quite a few well-documented facts about the effects of afternoon napping in both adolescents and adults from searching in the medical literature.

For example, napping after learning enhances memory consolidation (Cellini N, et al)  while taking a nap before learning improves the ability to encode material (Mander BA, et al)

But the issue is most of these studies don’t distinguish nap patterns and their consequences between habitual and non-habitual nappers.

40%-60% of adolescences experience naps at least twice a week and are considered habitual nappers. we know that they experience stage1 more in their naps and subsequently feel more refreshed upon waking. in contrast, individuals who are less accustomed to napping have more N3 when they do nap  (Spiegel R. – Evans FJ, et al)

Also, the duration of a nap is an important matter. surprisingly it has been shown that a 20-minute midday nap which is comprising mainly stage 1&2 sleep has deleterious effect on non-habitual nappers learning a “cup and ball” motor memory task. Besides, these short naps didn’t improve motor learning for habitual nappers. (Milner CE, et al)

Moreover, McDevitt EA, et al made a comparison between a 90min full cycle of sleep and staying awake in the afternoon and found that the 90-min nap enhances perceptual learning in a meaningful amount.

From the aspect of sleep deprivation No wonder that studies suggest that  children aged 3-6 years who used to nap , experienced more wake interference when kept awake compared to those who don’t have the habit of napping . (Kurdziel L, et al)

Results:

Sleep patterns:

HN group has significantly less TIB(time in bed) on weekdays(mean ± SEM difference: 38.25 ± 9.16 min, t(89) = 4.177, p < 0.001) , weekends (mean ± SEM difference: 30.56 ± 13.85 min, t(89) = 2.206, p = 0.030) and on an average(mean ± SEM difference: 36.05 ± 7.78 min, t(89) = 4.631, p < 0.001) and also less average nocturnal TST(total sleep time) which is attributable to their later bedtimes on both weekdays and weekends.

This study found that  A 90 min midafternoon nap is beneficial to both habitual and non-habitual nappers long term memory,  based on picture encoding task( assessed by participants recognition scores at retrieval) and factual knowledge task

Picture encoding task (assess long-term episodic memory) results show that participants who napped after encoding have significantly higher “A” scores than those who remained awake. (F(1,87) = 4.038, p = 0.048)

Factual knowledge task (assess long-term semantic memory) obtained two striking findings .firstly, individuals who napped prior to learning had remarkably better performance. (F(1,87) = 12.405, p = 0.001)

 Secondly, habitual napper individuals had better long-term memory performance than the non-habitual group, regardless of their sleep condition in this task  (F(1,87) = 6.657, p = 0.012).

Habitual nappers gained another special bonus in a short-term hippocampal-dependent topographical memory task. results showed that naps have an additional restorative function for them.

Moreover, HNs in this study scored higher on non-verbal intelligence.

But the bad news for HNs is that they have to continue this habit constantly. otherwise, nap restriction causes deterioration in their performance such as special memory and hippocampal-dependent learning problems.

Importantly, non-habitual nappers were not disadvantaged by taking a full cycle 90-min nap for any of the tasks in this study.

Contact: Alireza sardaripour

Source: Ruth L F Leong, Nicole Yu, Ju Lynn Ong, Alyssa S C Ng, S Azrin Jamaluddin, James N Cousins, Nicholas I Y N Chee, Michael W L Chee, Memory performance following napping in habitual and non-habitual nappers, Sleep, Volume 44, Issue 6, June 2021, zsaa277, https://doi.org/10.1093/sleep/zsaa277

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