Why does my temperature fluctuate when I sleep?

Feeling too hot—or too cold—is not conducive to sleep. How many of us have stuck our legs out of the covers to cool down, only to wake in the middle of the night with freezing toes?

As we sleep, our body temperature changes throughout the night, and there’s an important reason for this. Body temperature is controlled by our circadian rhythm, with core temperature rising and falling by a few degrees over a 24-hour period. The brain uses these changes to help it regulate the sleep/wake cycle, signaling the body to release hormones that either wake us or send us to sleep.

The timing of these fluctuations is mainly governed by the changes in natural light over each 24-hour period. A few hours before waking up, your temperature rises slightly as dawn approaches. This triggers a release of the energizing hormone cortisol to help wake you up. Your temperature continues to rise, peaking around late afternoon, before dropping again when the light fades. This generates the release of the hormone melatonin, which winds down alertness and prepares you for sleep. Body temperature continues to drop and is at its lowest a few hours after you fall asleep—which accounts for those freezing toes at 2 a.m.!

Core body temperature can also fluctuate in response to the activities you engage in, illnesses, the temperature of the environment around you, and—for those who menstruate—their monthly cycle. If you are too hot before bed, it will be harder to fall asleep, as your body won’t release enough melatonin. If you are too cold in the early morning, your body won’t release enough cortisol—which may explain why we struggle more to get up on cold winter mornings.

For better-quality sleep, start by getting your room temperature right, and be aware of the impact of your bedtime routine on core temperature—a hot bath or very strenuous exercise could interfere with melatonin production and make it harder to drop off.

Highs and lows

Although the body’s core temperature fluctuates by only a couple of degrees, these small changes are enough to set off the hormonal signals that control our cycle of waking and sleeping.

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