What it isThe temperature of your body’s internal organs and blood, as opposed to your skin temperature. Core body temperature follows a circadian rhythm, peaking in the late afternoon around 37.5°C (99.5°F) and reaching its lowest point (the temperature nadir) around 4-5 AM.

Why it mattersCore body temperature rhythm is one of the most reliable markers of circadian phase. Its timing relative to your sleep schedule reveals whether your internal clock is aligned with your behavior.

Think of it like thisYour core temperature is like an internal thermostat on a 24-hour timer. It turns up the heat when you need to be active and dials it down to cool you for sleep.

Core body temperature (CBT) is the temperature of deep internal tissues and circulating blood, maintained within approximately 36.5–37.5°C through hypothalamic thermoregulation. CBT exhibits one of the most robust circadian rhythms in human physiology, with a sinusoidal oscillation of ~0.5–1.0°C amplitude: peaking in the late afternoon/early evening and reaching its nadir in the early morning (typically 04:00–05:00).

MechanismThe SCN master clock modulates thermoregulatory set points in the preoptic area of the hypothalamus. During the evening decline, the SCN promotes vasodilation of distal skin (hands, feet) via sympathetic withdrawal, increasing heat dissipation. This peripheral heat loss lowers core temperature and facilitates sleep onset. The CBT nadir coincides with maximum melatonin secretion, minimum cortisol levels, and the circadian trough in alertness.