If you’ve ever stayed up the whole night through, you may have noticed that you begin to feel cold in the hours just before morning. Supposing you normally go to sleep at eleven o’clock, on the night that you stay awake the shivers will hit you somewhere around four or five a.m. — just about the time when you might otherwise pull up an extra blanket and sleep for another few hours. Although people often assume that the shivers come at the coldest part of the night, the actual explanation is found within the body itself. Sleep is one way of conserving energy. During the night, when we are inactive, it isn’t necessary to generate as much energy as we need for our daily business of moving around, finding food, and so forth. So, instead of maintaining the same temperature all day, which would waste a lot of our energy, our body temperature rises and falls in accordance to what is needed. This kind of pattern is called a circadian rhythm, because it takes one day to complete before starting over. The circadian temperature rhythm continues to run, regardless of whether you yourself have decided to go to sleep or not. Your body, you might say, executes the same strategy for conserving energy during what were supposed to be your off-hours. Your internal temperature descends to its lowest point late at night when you would normally have been in deep sleep. This conservation-mode is experienced by you as a creeping sensation that the room is getting colder and colder, when in fact it is your own body that is generating less and less heat. Do this for several nights in a row, and your temperature rhythm will begin to adjust itself. But anytime you keep yourself awake when the body expects to be asleep, those early-morning shivers will return.