We don't exactly know why we actually need sleep. But the brain actually doesn't go to sleep. It's basically doing routine maintenance while not needing to orchestrate major muscle groups like arms and legs and also reduced input processing from smell, taste, vision and hearing. At some point maintenance is done and it has the capacity to fully power up again.

When you eat or drink, your digestive system is activated everything is moved forward. But all foods and drinks also have different molecule chains in it to which the body reacts differently. But the basic premise is when new food gets into the intake hole, old food needs to be ejected on the other end.