What makes a child nap? Most parents cherish toddlers’ naps as moments of respite and recharging, for parent and child alike; we are all familiar with the increased crankiness that comes when a nap is unduly delayed or evaded. But napping behavior has been somewhat taken for granted, even by sleep scientists, and napping problems have often been treated by pediatricians as parents’ “limit-setting” problems.

Now, researchers are learning that it is not so simple: napping in children actually is a complex behavior, a mix of individual biology, including neurologic and hormonal development, cultural expectations and family dynamics.

What parents usually want to know is simply how long a child should nap. That concern dates back a little over a hundred years: In the first decade of the 20th century, European experts published the original studies measuring the sleep patterns of children and promptly began worrying they were not getting enough sleep.

Today, 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.