Identifying Neural Signatures of Satisfaction of Sleep Need
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Sleep consists of multiple stages: S1, S2, slow-wave-sleep (SWS), and rapid eye movement (REM). Do these stages serve complementary, different functions, or the same function but with different magnitudes? During non-REM (S1àS2àSWS), the brain signal progressively slows and becomes larger: Slow-wave oscillation (SWO: 0-1Hz) and signal in delta band (1-4Hz) in non-REM are the most unlike wake and are more prominent following sleep deprivation and during SWS [1,2]. Low-frequency power is thought to be an indicator of sleep need being met. Our studies, based on overnight, scored sleep data and brain signal from 5000+ subjects (8000+ nights)  suggest low-frequency power in S2 and in SWS and SWS duration, but not S2 duration, indicate sleep need being met: increased delta energy in S2 means less SWS duration (rs=-0.9145, p<<.00001), but not less S2 duration, i.e. more efficient sleepers can fulfill sleep need with increased delta power without getting to SWS. In fact, increased S2 delta power means shorter S2 durations, i.e. with greater sleep need, an individual will transition from S2 into SWS quicker. Increased delta power in SWS means shorter S1,2 durations suggesting that sleep need is not met by increasing S1,2 durations. Within-subject analyses for subjects for whom a second night of data was recorded corroborate all our results, affirming our conclusion that delta power across non-REM sleep and SWS duration are indicators of sleep need. Thus, S1,2 appear to be transitional stages for satisfying sleep need and may not have a distinct function from SWS after all.