To truly understand the adolescent DSPS, you need to understand the biological factors behind it. Two processes—Process S, or the homeostatic sleep system, which refers to the buildup of sleepiness with increasing hours of wakefulness; and Process C, or the human circadian day-night timing system and clock-dependent alertness—are involved in the regulation of the timing of sleep, and both are disturbed in the DSPS. In simple terms, Process S drives the need for sleep and Process C controls the timing of sleeping and wakefulness. How alert or sleepy you are depends on the sum of the interaction of these two processes.

Process S is a pretty straightforward process: The longer you’re awake, the greater “sleep need” you accumulate. However, except for a dip in alertness between 2:00 and 5:00 in the afternoon, you don’t notice the need to sleep very much until bedtime. That’s because it’s opposed by powerful circadian clock-dependent alerting signals from a region of the hypothalamus called the suprachiasmatic nucleus (see below). By bedtime, however, the sleep debt is overwhelming and causes you to pull up the comforter for some much-needed ZZZZs.

But what sends our bodies into sleep mode? It’s still a bit of a mystery. However, many scientists follow one of two theories: Either a chemical is building up in the brain and when there’s enough of it sleep results or a chemical is being depleted in the brain and when it’s used up the sleep curtain falls. Whichever is the case, Process S works like a clock, ticking off the time until you get drowsy enough to fall

Building sleep debt as the day progresses.