Cognitive effects of split and continuous sleep schedules in adolescents differ according to total sleep opportunity

Abstract

Study objectives: We compared the basic cognitive functions of adolescents undergoing split (nocturnal sleep + daytime nap) and continuous nocturnal sleep schedules when total sleep opportunity was either below or within the recommended range (i.e. 6.5 or 8 h).

Methods: Adolescent participants (age: 15-19 year) in the 8-h split (n = 24) and continuous (n = 29) sleep groups were compared with 6.5-h split and continuous sleep groups from a previous study (n = 58). These protocols involved two baseline nights (9-h time-in-bed [TIB]), 5 nights of sleep manipulation, 2 recovery nights (9-h TIB), followed by a second cycle of sleep manipulation (3 nights) and recovery (2 nights). Cognitive performance, subjective sleepiness, and mood were evaluated daily; sleep was assessed using polysomnography.

Results: Splitting 6.5 h of sleep with a mid-afternoon nap offered a boost to cognitive function compared to continuous nocturnal sleep. However, when total TIB across 24 h increased to 8 h, the split and continuous sleep groups performed comparably in tests evaluating vigilance, working memory, executive function, processing speed, subjective sleepiness, and mood.

Conclusions: In adolescents, the effects of split sleep on basic cognitive functions vary by the amount of total sleep obtained. As long as the total sleep opportunity across 24 h is within the recommended range, students may fulfill sleep requirements by adopting a split sleep schedule consisting of a shorter period of nocturnal sleep combined with a mid-afternoon nap, without significant impact on basic cognitive functions.

Clinical trial registration: NCT04044885.

Keywords: adolescents; cognition; continuous sleep; naps; split sleep; vigilance.

© Sleep Research Society 2020. Published by Oxford University Press on behalf of the Sleep Research Society.

Figures

Figure 1.

Protocol. In this 15-day protocol, both the 8-h split sleep group and the 8-h split continuous group had two adaptation and baseline nights (B1 and B2; TIB indicated by black bars = 9 h from 11:00 pm to 08:00 am). The first cycle of sleep opportunity manipulation lasted 5 nights (M11–M15) followed by two nights of recovery sleep (R11 and R12; TIB = 9 h). The second cycle consisted of three manipulation nights (M21–M23) and two recovery nights (R21 and R22). During the two sleep opportunity manipulation periods, the split sleep group had a nocturnal TIB of 6.5 h (00:15 am–06:45 am) and a 1.5-h nap opportunity between 2:00 pm and 3:30 pm, while the continuous sleep group had a nocturnal TIB of 8 h (11:30 pm–07:30 am). Asterisks indicate nocturnal sleep and daytime nap episodes with polysomnographic recordings. A cognitive test battery (yellow bars) was administered at 10:00 am, 4:15 pm, and 8:00 pm daily, except during the first and last days of the protocol.

Figure 2.

Sleep duration and macrostructure per 24-h period. The least square means and standard errors estimated with general linear mixed models are plotted for polysomnographically assessed (A) TST and duration of (B) N1, (C) N2, (D), N3, and (E) rapid-eye-movement (REM) sleep across each 24-h period separately for the 8-h split sleep group (red open circles and dotted line) and the 8-h continuous sleep group (red filled circles and solid line) during the second baseline night (B2), the sleep opportunity manipulation nights (M; gray shaded areas), and the recovery nights (R). ***p < 0.001, **p < 0.01, and *p < 0.05 for significant group contrasts.

Figure 3.

Markers of homeostatic sleep pressure. The least square means and standard errors of (A) N2 sleep latency and (B) SWA in the first hour of nocturnal sleep from N2 sleep onset are plotted for the 8-h split sleep group (red open circles and dotted line) and the 8-h continuous sleep group (red filled circles and solid line) from the second baseline night (B2) to the first and second cycles of sleep opportunity manipulation (M; gray shaded areas) and recovery (R). ***p < 0.001, **p < 0.01, and *p < 0.05 for significant group contrasts.

Figure 4.

Vigilance performance during a split or continuous sleep schedule when total TIB was below or within the recommended range. The numbers of lapses in the PVT are shown (A) averaged across the three tests each day, and separately for tests taken in the (B) morning, (C) afternoon, and (D) evening. PVT results are plotted after the last baseline night (day B2), during the first cycle of sleep opportunity manipulation (days M11–M15; gray shading) and after recovery nights (R11 and R12), to the second cycle of sleep manipulation (days M21–M23 in gray shading) and recovery sleep (R21). Observations for the 8-h split sleep group are shown in red open circles and dotted lines, while those for the 8-h continuous sleep group are illustrated in red filled circles and solid lines. For comparison, performance in a 6.5-h split sleep group (blue open circles and dotted lines) and a 6.5-h continuous sleep group (blue filled circles and solid lines) from a previous study [13] are also presented. The least square means and standard errors estimated with general linear mixed models are plotted. ***p < 0.001, **p < 0.01, and *p < 0.05 for significant contrasts between the split and the continuous sleep groups (red for the two 8-h sleep groups and blue for the two 6.5-h sleep groups).

Figure 5.

Other neurobehavioral functions during a split or continuous sleep schedule when total TIB was below or within the recommended range. The least square means and standard errors estimated with general linear mixed models are plotted for the daily average in (A) the number of correct responses in the MAT and the SDMT as measures of speed of processing, (B) A’ in the 1- and 3-back tasks as measures of working memory/executive function, (C) the score on the KSS as a measure of subjective sleepiness, and (D) the positive and negative affect scores on the PANAS. Data on the last baseline day (B2), as well as during the sleep opportunity manipulation periods (M; gray shading) and the recovery periods (R) are plotted in red open circles and dotted lines for the 8-h split sleep group and red filled circles and solid lines for the 8-h continuous sleep group. For comparison, performance in a 6.5-h split sleep group (blue open circles and dotted lines) and a 6.5-h continuous sleep group (blue filled circles and solid lines) from a previous study [13] also illustrated. ***p < 0.001, **p < 0.01, and *p < 0.05 for significant contrasts between the split and the consolidated sleep groups (red for the two 8-h sleep groups and blue for the two 6.5-h sleep groups).