September 15, 2025
Circadian Biology: The Basics That Most Guides Skip
The circadian system controls far more than sleep timing. A primer on the core mechanisms, why light is the dominant zeitgeber, and what happens when the system is misaligned.
Circadian BiologyThe Master Clock
The suprachiasmatic nucleus (SCN) in the hypothalamus functions as the master circadian clock in mammals. It receives direct light input via the retinohypothalamic tract and uses this to synchronise peripheral clocks throughout the body — in every organ and most tissues.
Why Light Is Primary
Light is the dominant zeitgeber (time-giver) because it is the most reliable environmental signal for time of day. Specialised retinal ganglion cells containing melanopsin are particularly sensitive to short-wavelength (blue) light and feed directly into the SCN. These cells are distinct from the rods and cones used for vision.
Peripheral Clocks
The liver, pancreas, gut, immune system, and cardiovascular system all have their own clocks that are synchronised by the SCN and by feeding signals. Eating at the wrong circadian phase — particularly late at night — can desynchronise these peripheral clocks from the master clock. This is one mechanism by which shift work increases metabolic disease risk.
Social Jet Lag
Social jet lag refers to the discrepancy between biological sleep timing (chronotype) and socially demanded sleep timing. The average discrepancy in the working population is about 1.5 hours. Chronic social jet lag is associated with higher BMI, worse mood, and poorer cognitive performance — independent of total sleep duration.
