“brain scans revealed a complicated interaction between two basic biological processes: the body’s central “circadian rhythm,” which pushes people to be awake and active during daylight, and wind down when it gets dark; and “homeostatic sleep drive,” which pressures people to go to bed when they’ve been awake too long.
The circadian rhythm is like a clock, while the sleep drive is like an hourglass, explained Dr. Charles Czeisler, a professor of sleep medicine at Harvard Medical School in Boston. He wrote an editorial that accompanied the study.
The sleep drive is an hourglass, he said, because the pressure to knock off gradually builds the longer you’re awake.
The circadian clock, on the other hand, determines the timing of your sleep and wake cycles by responding to light and darkness.
That’s why, if you stayed up from 7 a.m. until 7 a.m. the next morning, you won’t sleep the day away to make up for it, Czeisler explained. You’ll drop off, but only for a few hours, he said, because your “internal alarm clock” will go off.”
Read more at Brain Relies on Two Timekeepers for Sleep