Which physiological metric fluctuates according to the circadian rhythm?

Core body temperature is known to fluctuate according to the circadian rhythm, which is the body's internal clock that regulates various physiological processes in a roughly 24-hour cycle. Typically, core body temperature is at its lowest during the night and peaks in the late afternoon to early evening. This fluctuation is tied to the body's sleep-wake cycle and is influenced by factors such as light exposure and melatonin secretion.

In the context of aviation medicine, understanding the circadian rhythm and its effect on core body temperature can be essential for managing fatigue and optimizing performance for pilots and aircrew, especially during long flights where sleep patterns may be disrupted.