A delicate machine, the body subtly reacts to the environment around it without you even noticing most of the time. Sometimes, though, the effect of stimuli like cold can produce an alteration in the body that is more noticeable. An example of this is the sleepiness that a person starts feeling when she is very cold. It is important to note, though, that the existence of a correlation between cold and sleepiness doesn't mean that one caused the other.
Temperature homeostasis refers to the ability of the body to set a "thermostat" of between 36 degrees Celsius (96.8 degrees Fahrenheit) and 38 degrees Celsius (100.4 degrees Fahrenheit). When your body starts losing heat and your core temperature starts dropping, 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 35 degrees Celsius (95 degrees Fahrenheit), you are in mild hypothermia, which can produce sleepiness.
The normal body temperature range is important as it is the optimal temperature for essential biochemical reactions. As your body temperature drops, your brain doesn't work as efficiently as before. This causes issues like slowed reaction times, impaired judgment and tiredness. This occurs after only 1 degree Celsius drop in temperature from the normal range. The signs are subtle, and the person suffering from this mild hypothermia may not understand that he is becoming hypothermic from the cold. The drop in temperature occurs gradually, so the tiredness creeps up on the affected person. Common situations in which people suffer mild hypothermia include standing outside in cold temperatures all day (stallholders at farmers markets) or taking long journeys on a motorcycle in cold weather.
Moderate and Severe Hypothermia
When a person is in the first stages of hypothermia, he shivers at a relatively normal rate. In moderate hypothermia, at a core temperature of less than 35 degrees Celsius (95 degrees Fahrenheit), though, the shivering becomes violent and the tiredness worsens. The affected person also becomes confused and clumsy. At under 32 degrees Celsius (89.6 degrees Fahrenheit), the cold makes the person so sleepy that she is unable to move and slips 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 (a natural variation in temperature and sleepiness over 24 hours). The tiredness is not causing the cold, and the cold is not causing the tiredness, but rather the person's body clock has naturally caused the temperature of the body to drop. This generally happens in the early hours of the morning, which explains the presence of both cold and sleepiness at the same time. In addition, people feel colder when they are lying down and are more likely to be lying down when they feel sleepy.