Nocturnal heat impairs sleep architecture and autonomic activity in young men, with BMI-dependent effects on sleep

Elevated nocturnal temperatures impair sleep and cardiac autonomic activity. However, it remains unclear whether body mass index (BMI) modulates thermal sleep disruption. This study examined the effects of a single night of heat exposure (30°C vs. 22°C) on sleep architecture, EEG spectral activity, and autonomic regulation in young men with normal weight (NW) or overweight (OW).

Eighteen men aged 20-40 years (9 NW, 9 OW) underwent overnight polysomnography under thermoneutral (22°C) and warm (30°C) conditions in a controlled within-subject temperature-comparison design. Sleep architecture, EEG activity, and autonomic regulation were analyzed using linear mixed models (temperature × group). Subjective sleep quality was assessed via a visual analog scale.

Exposure to 30°C reduced total sleep time, sleep efficiency, and overnight cardiac parasympathetic activity in both groups. Sleep architecture showed BMI-dependent effects: individuals with NW exhibited heat-sensitive reductions in average slow-wave sleep (SWS) period duration and increased N1%, whereas individuals with OW showed shorter average SWS periods at 22°C, with no further heat-related reduction at 30°C. EEG spectral power showed increased N3 theta and alpha activity in individuals with NW, with no corresponding changes in individuals with OW. Sleep initiation measures in individuals with OW showed non-significant trends toward longer sleep onset latency and reduced wake-to-N1 transitions.

Nocturnal heat impairs autonomic activity similarly regardless of BMI, yet sleep disruption takes distinct forms. Deep-sleep continuity was heat-sensitive in individuals with NW, whereas individuals with OW showed shorter average SWS periods under thermoneutral conditions and suggestive alterations in sleep initiation under heat. BMI may therefore shape not whether, but how, heat-related sleep disruption is expressed.