Deep sleep homeostatic response to naturalistic sleep loss

Abstract Introduction Investigations of sleep homeostasis often involve tightly controlled experimental sleep deprivation in service of understanding mechanistic physiology. The extent to which the deep sleep response to recent sleep loss occurs in naturalistic settings remains under-studied. We tested the hypothesis that a homeostatic increase in deep sleep occurs on the night following occasional short duration nights that arise in naturalistic settings. Methods We analyzed sleep staging data in participants who provided informed consent to participate in the Apple Heart and Movement Study and elected to contribute sleep data. The analysis group included n=44,564 participants with at least 30 nights of sleep staging data from Apple Watch, from November 2022 to May 2023, totaling over 5.3 million nights. Results Short nights of sleep that were >=2 hours shorter than each participant’s median sleep duration occurred at least once in 92.9% of the cohort, most often in isolation (<7% of instances were consecutive short nights), and with a median duration of just over 4 hours. We observed that the amount of deep sleep increased on the subsequent night in proportion to the amount of sleep loss on the preceding short night, in a dose response manner for short night definitions ranging from 30 minutes to >=3 hours below the within-participant median sleep duration. Focusing on short nights that were at least 2 hours below the median duration, we found that 58.8% of participants showed any increase in subsequent deep sleep, with a median increase of 12% (absolute increase of 5 minutes). In addition, the variability in deep sleep after short nights markedly increased in a dose response manner. The deep sleep homeostatic response showed little correlation to sleep duration, timing, consistency, or sleep stages, but was inversely correlated with deep sleep latency (Spearman R = -0.28). Conclusion The results provide evidence for homeostatic responses in a real-world setting. Although the deep sleep rebound amounts are modest, naturalistic short nights are a milder perturbation compared to experimental deprivation, and reactive behaviors potentially impacting sleep physiology are uncontrolled. The marked increase in variability of deep sleep amount after short nights may reflect unmeasured reactive behaviors such as caffeine or napping, which exert opposing pressures on deep sleep compared to the homeostat. The findings illustrate the utility of longitudinal sleep tracking to assess real-world correlates of sleep phenomenology established in controlled experimental settings.