Purpose of sleep

So we see that the brain is not dormant during the one-third of our lives that we spend asleep. Instead, our sleep is actually a continuous swirl of brain activity, with regular cycles of more intense activity alternating with less intense activity.

But why is sleep necessary at all? Clearly, the 8 or so hours of relative inactivity associated with sleep has some advantages for our bodies: it reduces our energy expenditures so that the body's resources can be mobilized for tissue repairs and growth. That's one reason why infants and young children sleep so much, and why older adults sleep less and less as they age.