Predominance of Distal Skin Temperature Changes at Sleep Onset across Menstrual and Circadian Phases

This study investigated how the interaction between the menstrual cycle, circadian phase, and scheduled sleep-wake cycles influences body temperature regulation. Specifically, the researchers aimed to determine if the known increase in core body temperature (CBT) during the midluteal (ML) phase—often associated with progesterone elevation—impairs heat loss mechanisms that are critical for sleep onset.

Eight healthy women were studied during their midfollicular (MF) and midluteal (ML) phases. Participants underwent a 72-hour ultradian sleep-wake procedure involving 36 cycles of alternating 60-minute wake episodes and 60-minute nap opportunities under controlled constant conditions. The researchers monitored core body temperature, distal skin temperature (DT) at the hands and feet, and melatonin levels. They analyzed the distal-core temperature gradient (DCG) to assess peripheral vasodilation and heat loss.

While CBT was consistently higher during the ML phase compared to the MF phase, this did not translate into a reduction in peripheral heat loss. The study found that DT and DCG exhibited rapid, large increases at the onset of every nap, regardless of whether the participant was in the follicular or luteal phase. These distal heat loss mechanisms were solely dependent on the circadian phase and the timing of the sleep attempt, rather than the menstrual phase. The researchers concluded that even with the elevated baseline core temperature in the luteal phase, the body effectively maintains these heat loss responses, which likely explains why sleep propensity remains comparable between menstrual phases.

Understanding these thermoregulatory dynamics provides insight into the physiological basis of sleep disturbances reported by some women during different menstrual phases. By confirming that distal heat loss is robust across the menstrual cycle, this study supports the thermophysiological cascade model, which suggests that peripheral vasodilation is a fundamental requirement for sleep initiation. These results help clarify that menstrual-related temperature increases do not inherently damage the mechanisms needed to initiate sleep.