Perhaps the reason for circadian rhythms in humans is simpler: They help you sleep, and sleep is good for you.
When you lay your head down and nod off to the feather ball, your body is restoring itself. That includes basic upkeep and repair like muscle growth, tissue maintenance, protein production and the release of growth hormones. Those hormones help children develop naturally — exhibit A in the case against little Johnny staying up to watch Jimmy Fallon — but also play a key role in helping adults rebuild tissue over time. In fact, it's believed that some of these functions only happen during sleep hours. Animals deprived of sleep will lose all immune function and die in just a few weeks [source: Harvard Medical School].
If you've ever popped out of bed after a nice long slumber and felt mentally refreshed, it's probably not just because you spent the night dreaming about being fanned and fed grapes by models poolside at an Italian villa. Sleep helps humans restore their mental energy and cognitive functions that often get tapped out during waking hours [source: Harvard Medical School]. Our circadian rhythms naturally make us sleepy at night.