A delicate machine, the body subtly reacts to the environment without you even noticing most of the time. Sometimes, though, the effect of stimuli like cold can produce changes in the body that seem more apparent. For example, some people claim to feel like nodding off when they get chilly. However, a correlation between being cold and feeling sleepy doesn't necessarily equal causation.
TL;DR (Too Long; Didn't Read)
There may be a correlation between cold temperatures and feeling sleepy, but being cold doesn't technically cause tiredness. However, if you happen to be experiencing hypothermia, the condition may gradually make you feel fatigued and eventually lead to unconsciousness and coma.
Temperature homeostasis is the body's ability to maintain a temperature between 96.8 degrees Fahrenheit and 100.4 degrees Fahrenheit. When your body starts losing heat and your core temperature drops, you automatically start shivering and develop a strong instinct to move somewhere warmer. Most often, being cold is a minor issue. But if your core temperature drops to below 95 degrees Fahrenheit, you may be experiencing mild hypothermia, which can cause sleepiness.
The normal body temperature range is optimal for essential biochemical reactions. As your temperature drops, even by only a couple degrees Fahrenheit, your brain doesn't work as efficiently. You might experience issues like slowed reaction time, impaired judgment and tiredness. The signs are subtle, and a person suffering from this mild hypothermia may not understand it's happening. The temperature drop occurs gradually, so the sleepiness creeps up. Common situations in which people may experience mild hypothermia include standing outside in cold temperatures all day or taking long journeys on a motorcycle in cold weather.
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Moderate and Severe Hypothermia
When a person is in the first stages of hypothermia, they shiver at a relatively normal rate. In moderate hypothermia, at a core temperature of less than 95 degrees Fahrenheit, the shivering becomes violent, the tiredness worsens and the person becomes confused and clumsy. At under 89.6 degrees Fahrenheit, the person becomes so sleepy that they're unable to move, and they slip off into unconsciousness and coma.
Sometimes people may blame feeling sleepy on being cold, when in fact, the chill and the tiredness are due to a circadian rhythm. This is a natural variation in temperature and sleepiness over 24 hours. The tiredness and the coldness do not cause one another in this case. Instead, the person's body clock has naturally caused body temperature to drop. This generally happens in the early hours of the day. You might chalk it up to the morning chill and a bit of grogginess.
Also, people generally feel colder when they're lying down, and they're more likely to be lying down when they feel sleepy. So, clearly, there are a few coincidences that may be at play.