Circadian clues to sleep onset mechanisms.
Level 1 - systematic review of randomized trials
Meta-analysis of controlled constant routine laboratory experimental studies.
PubMed 11682282 · doi:10.1016/S0893-133X(01)00315-3
What was done
A meta-analysis of human studies conducted under controlled constant routine protocols followed by nocturnal sleep was performed. The analysis evaluated physiological and circadian predictors of sleep onset latency, including distal-proximal skin temperature gradient (indirect heat loss), core body temperature and its rate of change, heart rate, melatonin onset, and subjective sleepiness ratings, as well as the physiological shifts triggered by the "lights out" instruction.
What was found
The abstract reports no numerical values, effect sizes, or test statistics. Distal-proximal skin temperature gradient was identified as the best predictor of sleep onset latency compared to core body temperature, heart rate, melatonin onset, and subjective sleepiness. "Lights out" triggered rapid core-to-periphery heat redistribution (abrupt increases in skin temperature and decreases in heart rate) before sleep onset occurred, while sleep onset itself produced minor additional thermoregulatory changes.
Why it matters
This work identifies selective peripheral vasodilation and heat dissipation as the primary physiological mechanisms facilitating the transition into sleep, distinguishing circadian initiation from sleep maintenance.
Limits
The abstract does not disclose the number of pooled studies, total sample size, participant demographics, effect estimates, or confidence intervals. Constant routine laboratory protocols also minimize ecological variables (such as ambient temperature fluctuations and normal evening activity) that influence real-world sleep initiation.
Cited by
- context Core body temperature must drop by approximately 1 to 3 degrees to initiate and maintain deep sleep, and must rise by 1 to 3 degrees to wake up feeling alert.
- context Human body temperature must decrease by about 1 to 3 degrees to fall asleep and stay deeply asleep, and must increase by about 1 to 3 degrees to wake up.