The circadian rhythm or body clock is a biological process that occurs naturally on a 24-hour cycle (i.e. a full day). The word ‘circadian’ comes from the Latin circa (around) and dies (days).

The circadian rhythm is a 24-hour cycle of physiological, biochemical and behavioural processes that occur in the body, which are synchronised with the external light/dark cycles of the environment (known as Zeitgebers or ‘time givers’).

When there is a time change such as during air travel, the body needs time to adapt and re-synchronise the circadian rhythm with the external Zeitgebers of the new time zone, thus giving rise to the symptoms of jet lag.

For more information, see Sleep Physiology and Jet Lag.