Sleep
What is sleep and what is it for? The truth is that no one knows. We know that being deprived of sleep can kill you and so it is very important. It is often stated that diurnal animals sleep at night to keep them out of harm's way when nocturnal predators are about with superior night vision. It may well be the case but why have animals not evolved eyes or even two sets of eyes that would allow them to be active both at night and during the day? Surely this would be a great advantage. They would have twice the time to find food. After all mammals may have bettered reptiles by being more active since their worm-bloodedness allowed them to be active at lower temperatures. There is likely to be more to sleep than avoiding predators. There must be an imperative that trumps the advantage of continuous activity. Another theory is that the brain requires time to put in order all that it has learned. This is a possibility but fish sleep and do they learn enough to satisfy this theory?
As with many biological processes there may be more than one cause. People living in conditions of continuous light for experimental purposes still have a periodicity of wakefulness and sleep (the circadian rhythm), although it drift somewhat. It shows that sleep continues but does little to elucidate why the need evolved. Animals that need to save energy tend to sleep a lot. For example the wombat sleeps 16 to 18 hours a day. Predators have a better diet than herbivores and can afford to sleep more sleep more.
Maybe dreaming is important? We dream and we can be pretty sure that dogs do from observing our pets.
Maybe the dreaming spires of Oxford will come up with the answer some day or night.
Where would Shakespeare have been without sleep perchance to dream? How would the somnambulist keep fit?
Maybe butterflies don't sleep but they do rest between hurricanes.
The whole thing is a nightmare and I am growing tired so I think I will sleep on it.