Dave Munger explains studies connecting the sleep cycles of humans and other animals. For instance, reindeer might "be immune to jet lag because its circadian cycles are remarkably adaptable":
[For reindeer circadian cycles] are almost completely absent for much of the year, when the polar animals live in 24-hour daylight (or darkness, in the winter) since there’s no difference between day and night, a daily clock is not necessary. ...
Whether we’re adapting to jet lag or working the night shift, many humans might feel that circadian rhythms are more of a bother than they are worth. But think of it this way: If humans all slept and woke at random times, then it might be even more difficult for us all to get along. Without the dominant day-night cycle guiding our daily lives, how would we ever agree on times for meetings, meals, and other social activities? On the other hand, maybe that would be preferable to that annual ritual most of us share: The struggle of adapting to Daylight Saving Time.