Effects of Prior Time Awake and Time Asleep on Sleep Structure: Analyses across Forced Desynchrony Protocols with Different Sleep/Wake Cycle Durations

Introduction : Sleep timing and structure are affected by prior wake duration, length of time in current sleep episode and circadian phase. To investigate the first two effects independent of circadian phase, we conducted a meta-analysis of sleep recorded during forced desynchrony (FD) protocols with different prior wake and sleep episode durations. Methods : Fifty healthy young (19-34 years; 16F) participants with no medical, psychological or sleep disorders and free of medications and caffeine were studied in inpatient protocols with sleep/wake cycle (T) durations and prior wake durations: T=20 hr (Wyatt 1999), T=28 hr (Gronfier 2007), T=42.85 hr (Wyatt 2004, Grady 2010) each with a 2:1 wake: sleep ratio, and T=42.85hr (Cohen 2010) with a 3.3:1 wake:sleep ratio. Sleep was scored using Rechtschaffen and Kales criteria. During FD, sleep episodes are distributed evenly across the full circadian cycle. The amount of each stage of sleep (~600 sleep episodes) was averaged within subject by length of time since scheduled sleep onset in 1-hr bins; this averaging minimized circadian effect. Mixed models were performed for the effects of prior wake and length of time since scheduled sleep onset for different metrics of Slow Wave Sleep (SWS; NREM stages 3+4), REM sleep, and Wake. Results : SWS decayed approximately exponentially across sleep episodes with the exception of a sharp decrease in the 2nd hr and a sharp rise in the 3rd hr corresponding to the occurrence of the first REM sleep episode. SWS in the 1st hour had a non-linear dose-response relationship to prior wake duration. The time constant of a saturating exponential fit to SWS was different for T=20 hr than other T-cycle lengths. REM sleep values were lowest in the 1st hour, highest in the 2nd hour and then stable at an intermediate level for remaining hours; there were no differences in the 1st hour by prior wake duration. These results potentially demonstrate SWS-REM competitive homeostatic interactions. Wake exhibited the highest values in the first hour, low values for the 2nd and 3rd hour, and monotonically rising values for the remainder of the sleep episode. The initial value, minimum and slope for Wake all had non-linear inverse dose response relationships with prior wake duration. Conclusion: There are non-linear dose-response effects of prior time awake and prior time asleep on sleep stages.