The light-dark cycle caused by the rising and setting of the sun is responsible for what are called circadian rhythms. These are biological processes that show a cycle of around 24 hours, the time between consecutive rising of the sun. Experiments have shown that without the external cue of sunlight, the circadian rhythms of organisms tend to break down. These can be reset by being re-exposed to these cues. Jet lag is due in part to a re-setting of the circadian rhythm, as you travel to a different time zone, your body has to become used to a different timing of night and day.