Benefits of napping in healthy adults: impact of nap length, time of day, age, and experience with napping.

Napping is a cross-cultural phenomenon which occurs across the lifespan. People vary widely in the frequency with which they nap as well as the improvements in alertness and well-being experienced. The systematic study of daytime napping is important to understand the benefits in alertness and performance that may be accrued from napping. This review paper investigates factors that affect the benefits of napping such as duration and temporal placement of the nap. In addition, the influence of subject characteristics such as age and experience with napping is examined. The focus of the review is on benefits for healthy individuals with regular sleep/wake schedules rather than for people with sleep or medical disorders. The goal of the review is to summarize the type of performance improvements that result from napping, critique the existing studies, and make recommendations for future research.

CNS arousal and neurobehavioral performance in a short-term sleep restriction paradigm.

Few studies have investigated waking electrophysiological measures of arousal during sleep restriction. This study examined electroencephalogram (EEG) activity and performance during a 96-hour laboratory protocol where participants slept a baseline night (8 h), were randomly assigned to 3-, 5-, or 8-hour sleep groups for the next two nights sleep restriction (SR1, SR2), and then slept a recovery night (8 h). There were dose-dependent deficits on measures of mood, sleepiness, and reaction time that were apparent during this short-term bout of sleep restriction. The ratio of alpha to theta EEG recorded at rest indicated dose-dependent changes in CNS arousal. At 9:00 hours, both the 3- and 5-hour groups showed EEG slowing (sleepiness) during restriction, with the 3-hour group exhibiting greater deficits. Later in the day at 13:00 hours, the 5-hour group no longer exhibited EEG slowing, but the extent of slowing was more widespread across the scalp for the 3-hour group. High-frequency EEG, a measure of effort, was greater on the mornings following sleep restriction. The 5-hour group had increased beta EEG at central-parietal sites following both nights of restriction, whereas the 3-hour group had increased beta and gamma EEG at occipital regions following the first night only. Short-term sleep restriction leads to deficits in performance as well as EEG slowing that correspond to the amount and duration of sleep loss. High-frequency EEG may be a marker of effort or compensation.

Sensory gating impairments in poor sleepers during presleep wakefulness.

The neurocognitive model of insomnia predicts information processing deficits in poor sleepers. There is some evidence for deficits in later cognitive processing, but earlier sensory processing remains to be investigated. Paired-click stimuli were delivered to good and poor sleepers in a single night. P50 amplitude to stimuli provided an index of sensory gating in presleep wake, rapid eye movement sleep and stage 2 sleep. Poor sleepers exhibited sensory gating impairments during wake. For both groups, gating was intact in rapid eye movement sleep but absent in stage 2 sleep. These data show that poor sleepers experience enhanced sensory processing in the waking period before sleep. Further study is needed to explore sensory gating in chronic primary insomnia, sleep maintenance insomnia, and across multiple recording nights.

Physiological arousal and attention during a week of continuous sleep restriction.

Waking brain physiology underlying deficits from continuous sleep restriction (CSR) is not well understood. Fourteen good sleepers participated in a 21-day protocol where they slept their usual amount in a baseline week, had their time in bed restricted by 33% in a CSR week, and slept the desired amount in a recovery week. Participants slept at home, completing diaries and wearing activity monitors to verify compliance. Each day participants completed an RT task and mood and sleepiness ratings every 3 h. Laboratory assessment of electrophysiology and performance took place at the end of baseline, three times throughout the CSR week, and at the beginning of recovery. Participants reported less sleep during CSR which was confirmed by activity monitors. Correspondingly, well-being and neurobehavioural performance was impaired. Quantitative EEG analysis revealed significantly reduced arousal between the 1st and 7th days of restriction and linear effects at anterior sites (Fp2, Fz, F8, T8). At posterior sites (P4, P8), reductions occurred only later in the week between the 4th and 7th nights of restriction. Both the immediate linear decline in arousal and precipitous drop later in the week were apparent at central sites (C4, Cz). Thus, frontal regions were affected immediately, while parietal regions showed maintenance of function until restriction was more severe. The P300 ERP component showed evidence of reduced attention by the 7th day of restriction (at Pz, P4). EEG and ERPs deficits were more robust in the right-hemisphere, which may reflect greater vulnerability to sleep loss in the non-dominant hemisphere.

Habitual napping moderates motor performance improvements following a short daytime nap.

The effect of napping on motor performance was examined in habitual and non-habitual nappers who were randomly assigned to a nap or reading condition. Motor procedural learning and auditory discrimination tasks were administered pre- and post-condition. Both groups reported improved alertness post-nap, but not post-reading. Non-habitual nappers fell asleep faster and tended to have greater sleep efficiency, but did not differ from habitual nappers on other sleep architecture variables. Habitual nappers had greater alpha and theta EEG power in stage 1, and greater delta, alpha and sigma power in stage 2 sleep. Motor performance deteriorated for non-habitual nappers who napped, but improved for all others. The number of sleep spindles and sigma power (13.5-15 Hz) significantly predicted motor performance following the nap, for habitual nappers only. Results indicate that motor learning was consolidated in a brief nap and was associated with stage 2 spindles, but only for those who habitually take naps.

Frontal electroencephalogram alpha asymmetry during sleep: stability and its relation to affective style.

Electroencephalogram (EEG) alpha (8-12 Hz) asymmetries were collected from the mid-frontal and central regions during presleep wakefulness and Stage 1, Stage 2, and rapid eye movement (REM) sleep in 11 healthy right-handed participants who were free of psychiatric, neurological, and sleep problems. The authors found significant correlations between presleep wakefulness and different stages of sleep in the frontal, but not central, EEG alpha asymmetry measure. The strongest correlation was between presleep waking and REM sleep, replicating and extending relation earlier work to a normal population. The high degree of association between presleep waking and REM sleep may be a result of high cortical activation common to these states and may reflect a predisposition to different styles of emotional reactivity.

Waking quantitative electroencephalogram and auditory event-related potentials following experimentally induced sleep fragmentation.

STUDY OBJECTIVES: Experimental sleep fragmentation involves inducing arousals by administering intrusive auditory stimuli throughout the night. It is intended to model the frequent and periodic disruption experienced in common sleep disorders. Sleep fragmentation leads to daytime sleepiness, although evidence of performance impairment has been inconsistent. The purpose of this study was to investigate brain physiology associated with this level of sleep disruption. Specifically, quantitative analysis of electroencephalography was carried out, and auditory event-related potentials were recorded during daytime performance assessment following sleep fragmentation in good sleepers. DESIGN: Participants spent 4 consecutive 24-hour periods in the laboratory. On nights 2 and 3, sleep was fragmented using auditory stimuli that were delivered with increasing intensity until an arousal was noted. This design aimed to investigate the cumulative effects of sleep fragmentation on daytime functioning. SETTING: Data were collected in a sleep research laboratory during a 96-hour protocol. PARTICIPANTS: Eight healthy adults (mean age = 33.25) with no sleep complaints. MEASUREMENTS AND RESULTS: During the day, participants performed a 40-minute computerized test battery at 2-hour intervals (9:00 am -7:00 pm). The battery was presented in a fixed order and included measures of mood, sleepiness, reaction time, and serial addition or subtraction. Results indicated that subjective sleepiness and mood were impaired following sleep-fragmentation nights, compared to both baseline and recovery conditions. No performance deficits were apparent. The alpha:theta ratio, reflecting relative slowing of the electroencephalogram, was dramatically reduced following the second night of sleep fragmentation. Reductions in N1 amplitude of the event-related potentials indicated that attention was impaired with respect to early encoding processes following sleep fragmentation. CONCLUSIONS: Electroencephalographic and event-related potentials data illustrate impairment in information-processing capabilities associated with reduced arousal elicited by experimental sleep fragmentation. This subtle degree of sleep disruption, where total sleep time is not reduced, leads to sustained impairment in alertness and attention.