The question
Why do you feel sleepy at the same time each night, even without a clock?
The short answer
Nearly every cell carries a molecular clock that runs close to 24 hours, kept in time mainly by light. Shorter and longer rhythms run alongside it.
What science knows
Established ScienceIn mammals a small brain region, the suprachiasmatic nucleus, acts as a master clock and is reset each day chiefly by light reaching the eyes. The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for uncovering the genes behind these clocks.
Kept away from daylight, most people’s rhythm drifts slightly longer than a day; one careful study measured an average near 24.2 hours.
The pattern
Established ScienceBiological rhythms are grouped by their period:
| Rhythm | Period | Example |
|---|---|---|
| Ultradian | Under 24 hours | Sleep cycles of roughly 90 minutes |
| Circadian | About 24 hours | Sleep and wake, body temperature, melatonin |
| Infradian | Longer than a day | The menstrual cycle, commonly 21 to 35 days |
What is still uncertain
Active ResearchWhether a 90-minute rest-activity rhythm continues through the waking day, as Nathaniel Kleitman proposed, has mixed evidence. How much artificial light and screen time shift modern body clocks is an active field.
Why humans find it interesting
Historical FactIn 1729 the astronomer Jean-Jacques d’Ortous de Mairan noticed that a mimosa plant kept opening and closing its leaves on schedule even in darkness: one of the first hints that living things keep their own time.
CUE perspective
CUE PerspectiveCUE is about timing, and the body is proof that timing is real: the same task can feel easy at one hour and hard at another.
CUE’s daily readings use a separate, symbolic framework, not biology. But the idea that days have a rhythm worth working with is one science and CUE share.