If you’re lucky enough to be a seasoned traveller, you will no doubt have experienced one of the downsides of flying: jet lag. As if getting to the airport wasn’t stressful enough, you’re now craving breakfast in the middle of the night and fighting the urge to sleep when it’s nowhere near bedtime.
You don’t actually have to go on an exotic holiday to feel jet lagged. Believe it or not, ‘social jet lag’ is also a thing. If you stay up late then sleep in all weekend, you’ll experience a similar jarring feeling when your alarm goes off on Monday morning.
But why is this?
Sleeping and waking is one of the processes guided by your ‘body clock’. The term ‘body clock’ isn’t just an expression. Your body in fact has a ‘master clock’ situated in the hypothalamus in your brain which sends regulating signals throughout your body at different times of the day. These signals follow a twenty four hour cycle and are termed ‘circadian rhythms’. The word circadian comes from the Latin circa, meaning around, and diēm, meaning day.
To be classified as a circadian, a biological rhythm must meet three criteria:
- The rhythm has an endogenous (i.e. it comes from within the organism) free-running period that lasts approximately 24 hours. This means that the rhythm must continue in circumstances like constant darkness for 24 hours.
- The rhythm is entrainable. That is, the rhythm can be ‘reset’ by exposure to external stimuli such as light.
- The rhythm can be maintained in varied temperatures.
The first recorded observation of an endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729. He found that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica continued even when the plants were kept in constant darkness. 
It’s thought that circadian rhythms have evolved from when single celled organisms developed the ability to repair themselves at night after being damaged by UV light from the sun in the day.
The existence of your body clock is most noticeable when it’s wrong i.e. before you’ve had the chance to adjust to a new time zone. Even the one hour change for daylight savings can have an affect.
To reset these rhythms (see criteria 2 above) the brain needs the input of sunlight through the eyes. In fact, the area in the brain responsible for maintaining the circadian rhythms is found just above the point where the optic nerve fibers cross. The reset process isn’t instantaneous which is why it takes time to get over jet lag or shift work. Interestingly, in studies where humans have been kept in complete darkness, the daily cycles tend to lengthen to around 25 hours.
Research into circadian rhythms is ongoing, and we for one are excited by the prospect of to a solution to jet lag!
 de Mairan J. Histoire de l’Academie Royale des Sciences. ARS; Paris: 1729. Observation botanique; pp. 35–36