Suppression of the feeling of building sleepiness during the day by Process C: clock-dependent alertness. Note the slight increase in sleepiness at “siesta” time in the later afternoon.

asleep and counting the hours of sleep necessary to restore energy and alertness.

The problem with Process S and teens, though, is that teens don’t get as sleepy as quickly as adults and younger kids do; the curve for how sleepy teens get actually slows down and teens don’t accumulate as many “sleepiness points” as their younger siblings or parents. So a 15-year-old going through puberty can be up for the same number of hours as a 10-year-old or a 40-year-old but not feel as sleepy. That makes feeling awake well into the wee hours much easier for teens, but they still need nine hours of sleep to discharge their sleepiness.

Superimposed on this pattern lies the workings of Process C, in which our internal body clock regulates all of our biological processes. Process C causes us to feel more alert at certain times of the day even if, according to Process S, we’ve been awake long enough to have gained a large number of sleepiness points.

Our internal body clock is actually centered in a pinhead-sized nucleus called the suprachiasmatic nucleus, or SCN, that is deep in the hypothalamic region of the brain and receives environmental information about daylight and darkness via the retina. When that information

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