General Knowledge
Why do we sleep on a roughly 24-hour cycle even in a dark cave?
- The body keeps its own clock, close to but not exactly 24 hours
- We hear traffic
- We guess the time
The answer is The body keeps its own clock, close to but not exactly 24 hours. An internal clock, which daylight resets each morning.
Because the clock is internal. Nearly every cell carries a molecular oscillator built from proteins that switch their own genes off and back on over about a day, coordinated by a cluster of a few thousand neurons behind the eyes. Daylight does not create the rhythm; it resets it each morning.
Free of light cues, people drift - the natural period is a little over 24 hours for most, so isolated volunteers gradually go to bed later each day. Michel Siffre spent months underground in the 1960s and emerged convinced far less time had passed than had; a later stay left him with serious psychological effects.
The mechanism was worked out in fruit flies and won a Nobel in 2017. It explains jet lag - the body clock takes days to shift, roughly a day per time zone - and why eastward travel is harder, since lengthening a day suits a clock that runs long. It also explains why light at night matters: the resetting signal comes from a specific set of retinal cells most sensitive to blue, which is why a bright screen at midnight tells the clock it is morning.