Many mental disorders can disrupt the sweet embrace of a long, continuous sleep, including alcoholism, depression, Alzheimer’s and parenthood. And that’s bad news. We know that a good night’s sleep helps to solidify our memories of the previous day’s experiences. And according to a new study, we need a certain amount of continuous sleep for those benefits to kick in.

From an evolutionary point of view, it seems strange that we sleep for hours on end. Rather than leaving ourselves unresponsive and vulnerable for large chunks of time, why not simply sleep over several shorter fragments?

This is not an easy question to answer. Until recently, it has been all but impossible to break up the continuity of sleep without also affecting