Core body temperature (CBT) refers to the tightly regulated temperature of the body’s internal organs, which must remain stable for normal function. While often cited as 98.6°F (37°C), this internal measurement naturally fluctuates throughout a 24-hour cycle. These predictable shifts are intrinsic to the body’s daily rhythm. Understanding these temperature changes is central to the processes of alertness and rest.
The Core Answer: The Nightly Temperature Dip
Contrary to popular belief, core body temperature actually falls at night. This drop is a programmed physiological signal that precedes and accompanies sleep. The internal temperature varies by approximately 1 to 2 degrees Fahrenheit (0.5 to 1°C) over the course of a day.
The core temperature begins its decline shortly before a typical bedtime, a process that helps to initiate sleep. It reaches its lowest point, known as the temperature nadir, typically around 4:00 AM. After reaching this low point, the core temperature begins to rise again, which helps prepare the body for waking. This distinct pattern demonstrates that cooling is fundamental to the body’s nightly rest cycle.
How the Circadian Clock Regulates Core Temperature
The nightly temperature drop is actively managed by the body’s internal timekeeper, the circadian clock. This clock is primarily governed by the suprachiasmatic nucleus (SCN), a small region in the brain’s hypothalamus. The SCN coordinates the body’s cooling process by signaling a shift in the balance between heat production and heat loss.
The body cools primarily through vasodilation, the widening of blood vessels near the skin’s surface. This mechanism shunts warm blood away from the core and toward the extremities (hands and feet). The resulting increase in blood flow to the skin allows heat to dissipate more efficiently into the environment. This peripheral vasodilation is so closely linked to sleep onset that the warming of the hands and feet is considered a strong physiological predictor of falling asleep.
The Impact of Temperature on Sleep Quality
A slightly lower core temperature is not merely a consequence of sleep but an active requirement for both falling asleep and maintaining sleep quality. The body interprets the signal of a cooling core as a cue for rest, effectively “opening the sleep gate.” Conversely, a failure to shed heat, or a sustained elevated core temperature, can fragment sleep and reduce the amount of restorative deep sleep (slow-wave sleep).
For this reason, the temperature of the sleep environment significantly influences the natural cooling process. Sleep experts generally recommend setting the bedroom temperature to a cool range, typically between 60°F and 67°F (15.5°C to 19.5°C). A room within this range supports the body’s efforts to lower its own temperature without triggering shivering, which would generate counterproductive heat. Choosing breathable bedding and clothing is also important, as they prevent heat from being trapped against the skin and allow heat dissipation from the extremities.