00 · IN THREE MINUTES

The answer in three steps

  1. 1Clock genes and proteins form feedback loops that oscillate close to 24 hours.
  2. 2Special retinal cells report environmental light to the .
  3. 3The brain clock helps align sleep, hormones, temperature and organ clocks, while meals and activity provide additional timing cues.

01 · CELLS CAN KEEP TIME

Cells can keep time

Proteins encoded by clock genes build up, inhibit their own production and are later cleared. Delays in this loop create a self-sustaining rhythm that continues for a time even without external cues.

02 · LIGHT CORRECTS THE DRIFT

Light corrects the drift

The intrinsic period is not exactly 24 hours in everyone. Light detected by melanopsin-containing retinal cells changes clock-gene timing in the suprachiasmatic nucleus, shifting the phase each day.

03 · ONE CLOCK COORDINATES MANY CLOCKS

One clock coordinates many clocks

The central clock influences melatonin, cortisol, body temperature, autonomic activity and behaviour. Most organs also contain local oscillators that respond to systemic and local cues.

FIG. 02 · INTERACTIVEHow the evidence fits together

A conceptual model: the geometry shows relationships, not measured scale.

1

Cells can keep time

Proteins encoded by clock genes build up, inhibit their own production and are later cleared. Delays in this loop create a self-sustaining rhythm that continues for a time even without external cues.

feedback loop
A conceptual model: the geometry shows relationships, not measured scale.

04 · TIMING DETERMINES THE DIRECTION OF A SHIFT

Timing determines the direction of a shift

Light in the evening often delays the clock, while morning light often advances it. The phase-response curve explains why the same lamp can have different effects at different biological times.

05 · SYNCHRONY IS NEGOTIATED, NOT COMMANDED

Synchrony is negotiated, not commanded

Meals, exercise, social schedules and temperature can shift peripheral rhythms. Jet lag and shift work create temporary disagreement among clocks rather than simply turning one clock off.

06 · SOURCES AND EVIDENCE

Sources and evidence

The explanation follows the evidence chain below. Links open the original source.

  1. 01
    Circadian Rhythms

    Supports a specific claim or evidence boundary used in this article.

    INSTITUTIONAL EXPLAINER ↗
  2. 02
    The 2017 Nobel Prize in Physiology or Medicine

    Supports a specific claim or evidence boundary used in this article.

    INSTITUTIONAL EXPLAINER ↗
  3. 03
    Circadian clocks and metabolism

    Supports a specific claim or evidence boundary used in this article.

    PEER-REVIEWED REVIEW ↗
CHANGE LOG28 Aug 2026 · First five-language edition; claims, evidence boundaries and visual model checked.