A small cluster of neurons just above the place where the optic nerves cross keeps your body on a roughly 24-hour schedule. It is called the suprachiasmatic nucleus. Drag the clock hand and watch melatonin, alertness, and body temperature rise and fall together. Then switch on late-night screens and see what changes.
24-hour body clock
Drag the hand around the clock, or press Play.
The 24-hour rhythm
MelatoninAlertnessBody tempMelatonin with no screens
Set the timedrag the clock to begin
🌙 Melatonin-
⚡ Alertness-
🌡️ Body temperature-
Two forces set this pattern. One is the clock’s wake signal from the SCN, set by daylight. The other is sleep pressure, which builds the longer you are awake. Together they make up the two-process model of sleep (Borbély, 1982).