Our bodies experience a natural drop in temperature as we prepare for and enter sleep, which is a necessary physiological signal for rest. This cooling sensation is directly linked to a reduction in core body temperature (CBT), which must decrease for sleep to be initiated and maintained. The entire process of preparing for sleep is inherently tied to thermoregulation, the body’s method of controlling its internal temperature. This temperature fluctuation is a normal part of the 24-hour cycle and is not merely a side effect of lying still under a blanket.
The Internal Clock That Lowers Temperature
The timing of this nightly cool-down is managed by the body’s master biological clock, known as the circadian rhythm. This rhythm is centered in the suprachiasmatic nucleus (SCN), a small region located in the brain’s hypothalamus. The SCN acts as the body’s thermostat, dictating the schedule for temperature changes across the day and night.
The core temperature begins its descent, signaling the approach of bedtime, typically one to two hours before a person feels sleepy. This scheduled drop continues throughout the night, reaching its lowest point in the early morning hours, around 2 a.m. to 4 a.m., just before waking. The rapid rate of core temperature decline in the evening is a strong predictor of how quickly a person will fall asleep.
How the Body Dumps Excess Heat
The body actively sheds heat to achieve the necessary reduction in core temperature through a process called vasodilation. This mechanism involves widening the blood vessels, particularly those located in the distal extremities like the hands and feet. By increasing the blood flow to the skin’s surface in these areas, the body effectively moves warm blood away from the core.
The increased blood circulation to the skin allows heat to radiate outward into the cooler surrounding environment, turning the hands and feet into efficient heat-loss radiators. This heat transfer is the primary driver of the core temperature drop, which can be around 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) below the daytime peak. Studies have shown that this selective vasodilation in the distal skin regions is a better predictor of sleep onset than the overall core temperature itself.
The body also contributes to the cooling process by lowering its metabolic rate as sleep approaches. Reducing the internal generation of heat complements the outward heat transfer.
Thermoregulation During Sleep Stages
The body’s ability to regulate its temperature is not constant throughout the night but changes dramatically between the different sleep phases. During non-rapid eye movement (NREM) sleep, the body maintains some ability to regulate temperature, though the set point is lowered. The body can still initiate minor responses like shivering or sweating if the external environment becomes too hot or too cold.
However, thermoregulation is almost entirely suspended during rapid eye movement (REM) sleep, the stage associated with vivid dreaming. In this phase, the body essentially becomes poikilothermic, meaning it loses the ability to internally adjust to temperature changes and instead relies heavily on the environment. The brain stops initiating heat-conserving actions like shivering or heat-dissipating actions like sweating.
This loss of internal control during REM sleep makes a person highly vulnerable to the temperature of their surroundings. If the room is too cold, the sleeper may be woken by the thermal stress because the body cannot generate its own heat response.
The Impact of Your Bedroom Environment
The external environment plays a significant role in supporting or hindering the body’s natural cooling cycle. A cooler room temperature helps to facilitate the necessary core temperature drop, which is why experts often suggest an optimal bedroom temperature range. Maintaining a room temperature between 60 and 67 degrees Fahrenheit (15.6 to 19.4 degrees Celsius) is recommended for adults.
This cool environment supports the heat loss process, especially when the body’s internal thermoregulation is minimized during REM sleep. Bedding and sleepwear also interact with this process by providing insulation that creates a microclimate directly around the skin. Using bedding with appropriate insulation properties helps to maintain a stable, slightly warmer skin temperature, which supports the initial vasodilation necessary for heat loss from the core.