The Temperature of Falling Asleep : The Body’s Built-In “Thermal Rhythm”
1. Thermal Rhythm: The Body’s Built-In “Sleep Clock”
Have you ever had this experience: you feel sleepy at night, but your body feels too warm to settle down, so you toss and turn for a long time; or you wake up in the early morning with cold hands and feet and can no longer drift back into deep sleep? In fact, it is the body’s temperature changes that are “controlling” sleep.
The human body has a physiological mechanism known as the circadian temperature rhythm. During the day, in order to support physical activity and maintain wakefulness, core body temperature stays at a relatively high level, usually around 37.2–37.5°C. In the evening, as melatonin production increases, core temperature gradually begins to fall. When it drops into the “sleep zone” of 36.5–37°C, sleepiness naturally sets in. Once deep sleep begins, body temperature reaches its lowest point of the day. In the early morning, temperature slowly rises again in preparation for waking. 
In simple terms, a drop in body temperature is the body’s signal that it is time to sleep, while keeping body temperature stably low is what helps protect deep sleep. If this rhythm is disrupted — for example, if body temperature does not fall at night, falls too slowly, or rises too early in the morning — sleep is directly affected. The result may be trouble falling asleep, lighter sleep, frequent waking, and ultimately insomnia.
2. The “Abnormal” Body Temperature Patterns Seen in People with Insomnia
Many people with long-term insomnia have problems with temperature regulation. The most common patterns are:
1. Core body temperature stays too high at night, making it hard to fall asleep
Under normal circumstances, core temperature gradually decreases after 6 p.m. But for some people with insomnia, the drop is very slow. Even when it is time to sleep — for example, at 10 p.m. — their core temperature may still remain above 37.3°C, which is higher than the sleep zone. At that point, the body is still in “wake mode,” and the brain cannot fully relax, so sleepiness does not come naturally. Even if they lie in bed counting sheep or listening to white noise, they may still feel physically alert and mentally restless. Often, they do not fall asleep until very late at night, when temperature finally drops enough.
2. Large temperature fluctuations during the night lead to lighter sleep and frequent awakenings
Some people with insomnia can fall asleep, but their body temperature is unstable during the night. For example, core temperature may not fall low enough during deep sleep, or it may begin rising quickly around 2 or 3 a.m. These fluctuations disrupt the sleep cycle. Deep sleep that should last one to two hours may be shortened to only half an hour. Even small temperature shifts can trigger the brain’s “wake-up” signals, causing a person to wake repeatedly during the night without knowing why. After waking, they may feel exhausted, yet still be unable to return to deep sleep. 
3. Low temperature in the hands and feet makes sleep slower and lowers sleep quality
In addition to core temperature, the temperature of the hands and feet also affects sleep. During normal sleep onset, the temperature of the hands and feet rises above core temperature by about 1–2°C — for example, hand temperature may reach 32–33°C. This “warming of the extremities” helps the body release heat, which in turn promotes a drop in core temperature. But many people with insomnia have poor circulation in the extremities. Their hands and feet feel cold at night, with hand temperature below 30°C. As a result, heat dissipation is blocked, core temperature cannot fall smoothly, and it may take them more than 30 minutes longer than average to fall asleep. Even after they do fall asleep, poor heat regulation can make sleep lighter, leaving them with the feeling that they “did not sleep enough” when they wake up.
