Activity in every cell in our bodies follows a daily 24 hour rhythm or circadian cycle, which is governed by a master 'body clock' in the brain. This clock regulates our patterns of sleep, eating, alertness, body temperature, growth and many other biological functions.
The body clock adjusts our daily cycles based on external cues, known as Zeitgebers (meaning “time-givers”), of which daylight is the most important. For example, when it gets dark in the evening, the body clock increases the production of melatonin, a hormone which causes drowsiness and lowers body temperature. Levels remain high during sleep but taper off in response to light.
Jet lag arises when we expect our body to perform activities at times when it's prepared for rest or recovery, or when we try to sleep when the body clock is sending cues to drive alertness.
Some of the unpleasant symptoms of jet lag are due to fatigue. When we're sleep deprived, we can trigger two opposing systems – the parasympathetic nervous system and the sympathetic nervous system. The sympathetic nervous system drives the flight or fight response, which throws the body into high stress alert mode. The parasympathetic nervous system controls at-rest functions like digestion. Your symptoms will depend on which system reacts the most.
To reduce jet lag, we can actively manage the timing of exposure to Zeitgebers such as daylight, darkness and mealtimes to help adjust to a new time zone, and make time for good quality sleep.