"By adulthood, most of us have adopted the habit of sleeping through the night and staying awake for most or all of the day—but this may not be ideal for our mental health and is certainly not the only way people have slept throughout history. In somewhat the same way that hobbits in Tolkien's Middle Earth enjoy a first and second breakfast, people living without electricity in preindustrial Europe looked forward to a first and second sleep divided by about an hour of crepuscular activity. During that hour, they would pray, relieve themselves, smoke tobacco, have sex and even visit neighbors. Some researchers have proposed that people are also physiologically inclined to snooze during a 2 P.M. to 4 P.M. “nap zone”—or what some might call the afternoon slump—because the brain prefers to toggle between sleep and wake more than once a day. As far back as the first century B.C. the Romans regularly took midafternoon breaks, which they called meridiari from the Latin for midday. Under the influence of Roman Catholicism, noon became known as sexta (the sixth hour, according to their clocks), a time for rest and prayer. Eventually sexta morphed into siesta."
So reports Ferris Jabr in Scientific American (10/15/13)-- "Why Your Brain Needs More Downtime." The answer is that learning, memory, even our sense of identity is highly dependent on sleep, rest and downtime. Even simple daydreaming stimulates what is called the "default mode network" (DMN), which coordinates and integrates communication between disparate regions of the brain.
"Numerous studies have shown that meditation strengthens connections between regions of the default mode network, for example, and can help people learn to more effectively shift between the DMN and circuits that are most active when we are consciously fixated on a task. Over time expert meditators may also develop a more intricately wrinkled cortex—the brain’s outer layer, which is necessary for many of our most sophisticated mental abilities, like abstract thought and introspection. Meditation appears to increase the volume and density of the hippocampus, a seahorse-shaped area of the brain that is absolutely crucial for memory; it thickens regions of the frontal cortex that we rely on to rein in our emotions; and it stymies the typical wilting of brain areas responsible for sustaining attention as we get older.
"Just how quickly meditation can noticeably change the brain and mind is not yet clear. But a handful of experiments suggest that a couple weeks of meditation or a mere 10 to 20 minutes of mindfulness a day can whet the mind—if people stick with it. Likewise, a few studies indicate that meditating daily is ultimately more important than the total hours of meditation over one’s lifetime."