Personally, I think that most behaviours/attributes have evolved out of the basic laws of life (survive and reproduce) and of Physics, especially energy-related constraints. Based on that the most obvious basis for sleep is energy saving.

I believe that 'sleep' has been observed in animals as simple as jellyfish so any neuroscience-centred explanation is bound to miss the root cause, in my view. But, as often in evolution, the development of nervous systems may have taken advantage of pre-existing sleep patterns.