Life on earth has developed under the rhythmic alternations of (sun-)light and darkness. Unsurprisingly, many processes inside our body also follow a pattern that is repeated about every 24 hours. This pattern is called a ‘circadian’ rhythm, a name that is derived from Latin ‘circa diem’, which means ‘approximately one day’. For example, our body temperature, the secretion of certain hormones, but also many behaviours follow such a circadian pattern and the sleep-wake cycle is probably the most obvious one.

All these rhythms are brought about and orchestrated by a tiny clock that is ticking inside our brains. This clock is only about and not precisely one day, so how can we live in a 24-hour world without going off course?

To solve this problem, nature arranged for the internal biological clock to use other signals that tell the time – so called zeitgebers (from German: something that gives time). Among these, daylight is the most important signal that entrains the body clock to the earth's 24h day, but also bodily activity, food intake, and other factors contribute to a lesser extent.

Usually, we are unaware of our circadian rhythms, we simply take them for granted. That is, until they get messed up, as for example during jet lag. But you do not necessarily have to travel far, also people working night-shifts often suffer from circadian misalignment. Poor sleep and excessive sleepiness during the day are prevalent symptoms. Consequently, as you can imagine, mood will often suffer and those affected cannot call up their optimal performance capacity.

Fact: If you sat in a room in complete darkness without knowing what time it was, your brain would think a day lasts more than 24 hours and thus your daily rhythm would quickly shift.