Thermoregulation Cycling refers to the highly predictable, circadian rhythm in core body temperature (CBT), which is a key physiological output of the master clock. This cycle involves a gradual decrease in CBT beginning in the late afternoon, reaching its nadir in the early morning hours, followed by a gradual rise throughout the day. This rhythmic drop in temperature is a necessary signal for initiating and maintaining sleep. Optimal cycling is essential for sleep quality, metabolic rate regulation, and overall homeostatic balance. Disruptions to this rhythm, often due to external temperature or light exposure, can impair sleep and alertness.
Origin
The concept is fundamental to chronobiology, where the core body temperature rhythm has historically been one of the most stable and easily measured circadian markers. ‘Thermoregulation’ names the process of maintaining internal temperature, and ‘cycling’ emphasizes its rhythmic, 24-hour nature. The precise timing of the minimum temperature, the CBT minimum, is used as a reliable marker of the body’s internal clock phase.
Mechanism
The Suprachiasmatic Nucleus (SCN) drives this rhythm by controlling the balance between heat production and heat loss. In the evening, the SCN promotes vasodilation of peripheral blood vessels and decreases metabolic heat production, leading to heat dissipation and the subsequent drop in CBT. This temperature decline facilitates the initiation of sleep, while the morning rise in CBT acts as a wake-promoting signal, demonstrating a tight, clock-driven link between temperature and the sleep-wake cycle.
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