Nocturnal heat impairs sleep architecture and autonomic activity in young men, with BMI-dependent effects on sleep.
Level 2 - randomized trial
Controlled within-subject crossover physiological trial in humans
PubMed 42508230 · doi:10.1016/j.sleep.2026.109155
What was done
Eighteen men aged 20–40 years (9 normal weight, 9 overweight) underwent overnight polysomnography under thermoneutral (22°C) and warm (30°C) conditions in a controlled within-subject temperature-comparison design. Sleep architecture, EEG spectral power, and cardiac autonomic regulation were analyzed using linear mixed models (temperature × group), alongside subjective sleep quality evaluated by visual analog scale.
What was found
Exposure to 30°C reduced total sleep time, sleep efficiency, and overnight cardiac parasympathetic activity in both normal-weight and overweight groups (specific numerical values were not reported in the abstract). Sleep architecture effects differed by BMI: normal-weight men showed heat-induced reductions in average slow-wave sleep (SWS) period duration, increased N1 percentage, and increased N3 theta and alpha EEG activity. Overweight men exhibited shorter average SWS periods at 22°C with no further reduction at 30°C, no corresponding EEG changes, and non-significant trends toward prolonged sleep onset latency and fewer wake-to-N1 transitions.
Why it matters
This study suggests that while nocturnal heat disrupts autonomic regulation and sleep efficiency regardless of BMI, the specific manifestations of sleep architectural disruption differ by body composition.
Limits
The sample size was very small (n=18; 9 per group) and limited exclusively to young adult men, precluding generalization to women, older populations, or individuals with higher degrees of obesity. The abstract does not provide exact effect sizes, confidence intervals, or details on exposure randomization.
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- contradicts Heating the sleep environment toward the end of the night increases REM sleep, whereas cooling it at the beginning of the night increases deep sleep.