One thing that all animals do is sleep, yet there is still no clear idea of why we do it and why it is so necessary. Many of the possible explanations that I have heard before have had something to do with memory formation or something similar to that. Thus, it was quite interesting to read about another suggestion, which is something completely different. Apparently, there is a possibility that sleep, i.e. shutting the mind down to some extent, might be necessary to physically clean the brain of certain chemicals that build up there during it normal operations. Assuming, of course, that mouse brains are not special.
Puzzles of Sleep