Light. Especially sunlight, the exact effects of artificial light are not completely clear (or rather, it seems to have an effect, but it's not completely obvious which parts and how: this is part of the theory of why avoiding blue light in the evening can help with sleep but it's not actually been demonstrated that it works as an intervention).

People's daily clock is naturally (i.e if kept in a dim light constantly) slightly longer than a day (exactly how long varies by a few hours), but it becomes synchronised to changes in light level (there are actually specific light receptors in your eyes that do this, independent of vision). Exactly what 'phase' this clock has compared to the change in light levels varies from person to person, and throughout their life. There's a decent body of evidence that teenagers naturally shift towards being more delayed as they go through puberty. There are even disorders where it can become very far out of whack: people who will want to sleep very early or very late, or in the most extreme case (usually people who are totally blind in a way that also disrupts these specialised receptors, but also some people who have functioning sight), where the synchronisation doesn't work at all and their natural sleep cycle will drift freely with respect to the day-night cycle.