Slow nocturnal body cooling during sleep increases interbeat intervals and is tightly coupled to high-frequency heart rate variability in healthy men.

During nocturnal sleep, core body temperature (CBT) decreases and cardiac inter-beat intervals (IBI) increase. This study used a non-disturbing cooling intervention during sleep to investigate additional effects of CBT reduction on IBI and heart rate variability. Sleep on a high heat-capacity cooling gel mattress (HM) was compared to a low heat-capacity mattress (LM) in 32 healthy males (age 46 ± 4 years). Electroencephalography, electrocardiography, CBT, skin, mattress, and air temperatures were recorded under constant conditions. Power in the low- (LF) and high-frequency (HF) bands and the LF/HF ratio were calculated from one-minute IBI intervals. Sleep on HM led to greater CBT decline and showed increased IBI and HF in all sleep stages. Mediation analyses indicate HF is not directly influenced by CBT, but HF changes follow IBI, which mediates CBT effects on HF. HF and LF differ between sleep stages: HF is lower during rapid eye movement (REM) and higher during non-REM, independent of CBT or IBI. In sum, sleep on a cooling mattress decreases CBT, which increases IBI and HF, independent of sleep stages. Cooling-induced HF changes are mediated by IBI. In contrast, HF and LF vary with sleep stages, reflecting autonomic activity throughout sleep cycles, independent of thermophysiological changes.