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.