In classic isolation experiments, volunteers have spent weeks living in caves or bunkers with no sunlight, no clocks, and no way to know what time it was outside. Rather than drifting into a chaotic, random sleep schedule, most kept sleeping and waking on a remarkably regular cycle — usually close to but not exactly 24 hours. That's the signature of the circadian rhythm, an internal biological clock that keeps running on its own timetable even when every external cue for time has been stripped away.
A clock built out of neurons and genes
The body's master circadian clock lives in the suprachiasmatic nucleus (SCN), a small cluster of neurons in the hypothalamus, deep in the brain. Cells in the SCN generate their rhythm through a feedback loop of specific "clock genes" that switch each other on and off in a cycle lasting close to 24 hours, a genetic mechanism found, in some form, across an enormous range of organisms — not just humans, but many animals, plants, and even some microorganisms — suggesting a roughly day-length internal clock is an ancient and broadly useful evolutionary adaptation, not an unusual quirk of human biology specifically.
Free-running rhythms reveal the clock's true period
Under normal daily life, the SCN's clock is continuously reset, or "entrained," by external light exposure hitting the eyes, which keeps it synchronised to the actual 24-hour day. In the total absence of light and other external time cues — as in cave isolation studies — the clock instead "free-runs" on its own internal, genetically set period, which for most people turns out to be slightly longer than 24 hours, typically somewhere around 24 to 25 hours. That small daily drift is why participants in these studies gradually shift their sleep and wake times later and later across the weeks, cycling slowly out of sync with the actual clock outside, even while their own internal rhythm stays remarkably stable and regular on its own terms.
What we're still unsure about
The existence of the SCN as the body's master circadian clock, and the basic molecular feedback loop driving it, are well established in neuroscience. What remains more actively researched is exactly how this master clock coordinates with the many "peripheral clocks" found in other organs and tissues throughout the body, each running on similar molecular machinery but capable of drifting somewhat independently — and precisely how disruptions to that coordination, such as from shift work or chronic jet lag, translate into the specific long-term health effects researchers have observed, is still being worked out in detail.
This sits inside Sleep, Circadian Rhythms & the Brain, one of seven topics in Neuroscience, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.