I sleep at the wrong times
Brains regulate the sleep-wake rhythm. This biological clock is different in every individual and can be disrupted, for instance by sleeping in, travelling by air or working shifts. Some people’s biological clock deviates from the social rhythm by nature.
Sometimes, someone’s sleep-wake rhythm deviates so much, that it causes serious sleep problems and considerable problems functioning in daily life. The Center for Sleep Medicine has specific expertise on so-called circadian rhythm disorders.