It could be that we sleep to conserve energy. Our ancestors didn't have lights (except when they discovered fire), so they couldn't do anything at night. Maybe to conserve energy, they went to sleep. That is why people who don't sleep very much at night need to eat more food to replace the energy they lost because they weren't asleep.

 

It is very likely one function of sleep, but there is certainly more to it. First of all it is not only that we use less energy during sleep, but staying awake while being sleep deprived costs more energy than just being awake but not being sleep deprived. The energy cost associated with staying alive was calculated to be roughly two slices of bread (Jung et al. 2011 The Journal of Physiology, 589, 235-244). Considering other factors (e.g. influence of sleep on cognitive abilities) it is clear that sleep has far-reaching physiological effects beside conserving energy.