Cells at your hair roots keep their own daily clocks.

A hair root keeps time in its own cells

At the base of a growing hair, living follicular cells switch genes on and off in a roughly 24-hour rhythm. These circadian rhythms are part of the body’s internal timing system, which helps coordinate daily changes in physiology. The follicle’s clock is influenced by the body’s broader rhythms, including signals associated with the sleep–wake cycle.

When genes are active, cells produce RNA—the working copies used to make proteins. Because the mix of RNA changes over the day, a sample from hair-root cells can carry a time-stamped pattern. The strands themselves do not sense bedtime; the timing information is in the living cells attached to the roots.

Reading the rhythm, not a bedtime

Researchers can examine that RNA pattern and compare it with the daily timing of clock-related gene activity. The pattern may help indicate a person’s chronotype: their general tendency to feel alert earlier or later in the day. That makes follicle cells a potential window into internal timing, without relying only on someone’s recollection of their habits.

But a cellular clock is not a log of exactly when someone went to sleep. Circadian timing and sleep behavior are related, yet they are not identical: a person may stay up later than their body’s preferred schedule, for example. A follicle sample therefore offers clues about daily biological rhythms, not a precise record of an individual night.

The useful twist is that a routine-looking hair root can preserve a changing molecular pattern—one that reflects when the body’s clock is running early or late.