researchers believe that very young children take naps because so-called sleep pressure builds rapidly in their brains — that is, the need for sleep accumulates so quickly during waking hours that a nap becomes a biological necessity. It is not just a question of how much total sleep that children need in 24 hours. Possibly because of the intense synaptic activity that goes on in their highly active, highly connected brains, young children are less able to tolerate long periods of time awake.