A reader asks:

Why do I wake up at 4:30 every morning no matter what time I go to bed?

Answer:

We can make an intelligent guess. There are two main physiological factors that affect sleep timing: the internal circadian clock, and how long you stayed awake before you went to sleep.  Your circadian clock, or chronotype, may be set very early. (See what your chronotype is by anwering the Automated Morningness-Eveningness Questionnaire (MEQ) at cet.org.)

The second factor operates differently. During waking hours we build up a neurochemical sleep substance in our brain.  More of the substance builds up the longer we stay awake. After we go to sleep, the substance gradually disappears.

You may have a deficiency in this sleep substance. This would allow the circadian clock to wake you up at its appointed hour regardless of how long you stayed awake the previous day. In other words, because you do not have as much of the sleep substance as most people, your sleep substance would not be enough to counteract your internal clock. Your internal clock would thus have a stronger influence on you than the internal clock in other people, who can sleep later when their bedtime is delayed.

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