There is a theory that we spend part of the night working on the brain (paradoxical sleep) and the rest of the night working on the body (orthodox sleep). Presumably these two restoration jobs would be taken in turns so that at whatever time it is necessary for us to wake, we will have received some benefit from each of them. This is almost certainly an over-simplification but there is some evidence that fits quite neatly. For example, it has been shown that athletes whose lifestyle puts a great deal of strain on their bodies need more orthodox sleep. Children need a greater amount of both kinds of sleep to allow growth, especially paradoxical sleep in the early weeks when the gray matter of the brain is developing very quickly.