How come I wake up just before the alarm goes off?

Dear Cecil: How is it that people often wake up moments before their alarm goes off? I know this isn’t just me. I thought maybe the body gets accustomed to a set number of hours of sleep or maybe it responds to something like the sun being at a certain place. But that isn’t possible because you’d have to go to sleep exactly at the same time every night, and besides, the sun is in a different position each day. I could see that the body would rather wake up on its own than have a loud noise jarring it out of sleep, but that doesn’t explain how a sleeping person can know when to wake up. Help, Cecil. David D., N. Sheridan

Cecil replies:

We can chalk this up to two things, Dave: your natural body rhythms plus some classic Pavlovian conditioning. Scientists have long been interested in what are called “circadian rhythms” — i.e., cyclic fluctuations in various bodily functions, such as memory and manual dexterity, over the 24 hours of the day. Many of these rhythms are endogenous — that is, controlled internally rather than by by external factors like the sun. (Interestingly, if you’re isolated from the outside world and don’t know what time it is, oftentimes your body will adjust to a twenty-five hour cycle.) For our purposes, the most important function to be concerned with is body temperature, which is highest at midday and lowest in the early morning hours. (That’s why if you pull an all-nighter you’ll feel chilly around 4 AM, even if room temp stays constant.) Studies by Harvard researcher Charles Czeisler indicate that waking is strongly linked with a rise in body temperature, and that body temperature rises at about the same time each morning regardless of what time you got to bed. In other words, your internal alarm clock will wake you up even if you aren’t fully rested — which may account for the fact that people on irregular schedules sometimes complain they wake up tired.

But your biological alarm clock doesn’t instantly jolt you to a state of full alertness when morning comes around. Rather, your sleep becomes progressively lighter, and alternates with brief periods of wakefulness. Here’s where the Pavlovian conditioning comes in. You may not be consciously aware of this, but in virtually all mechanical and many electric clocks there’s a faint click or some other change in rhythm a short time before the alarm goes off. You’re quite capable of hearing this in your semi-wakeful pre-dawn state, and over time you associate it with the alarm that invariably follows. Since the alarm itself is so unpleasant, you unconsciously learn to snap to a state of full wakefulness as soon as you hear the click so you can shut the alarm off. If you were to buy a clickless electronic clock or clock radio, however, you’d be more likely to sleep straight through till the alarm.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.