According to the World Health Organization, depression is the leading cause of disability and the fourth leading contributor to the global burden of disease, projected to reach second place by 2020. There is also strong evidence that it is an independent risk factor for heart disease, and several studies show that prolonged depression is associated with selective and possibly permanent damage to the hippocampus, a region of the brain critical to memory and learning.
Add the fact that 2 percent to 12 percent of depressed people eventually commit suicide, and the [supposed evolutionary] “advantages” of depression suddenly don’t look so good....
What is natural, the thinking goes, is best. If we are designed to suffer depression in response to life’s ills, there must be a good reason for it, and we should allow it to take its painful and natural course.
But unlike ordinary sadness, the natural course of depression can be devastating and lethal. And while sadness is useful, clinical depression signals a failure to adapt to stress or loss, because it impairs a person’s ability to solve the very dilemmas that triggered it.
Even if depression is “natural” and evolved from an emotional state that might once have given us some advantage, that doesn’t make it any more desirable than other maladies. Nature offers us cancer, infections and heart disease, which we happily avoid and do our best to treat. Depression is no different.

'Later that day...participants performed a new round of learning exercises. Those who remained awake throughout the day became worse at learning[;] those who napped did markedly better and actually improved in their capacity to learn....
'[S]leep researchers have established that fact-based memories are temporarily stored in the hippocampus before being sent to the brain's prefrontal cortex[:] "It's as though the e-mail inbox in your hippocampus is full and, until you sleep and clear out those fact e-mails, you're not going to receive any more mail. It's just going to bounce until you sleep and move it into another folder"....
'Electroencephalogram[s]...indicated that this refreshing of memory capacity is related to Stage 2 non-REM sleep, which takes place between deep sleep (non-REM) and the dream state known as Rapid Eye Movement (REM). [The purpose of this stage has been unclear, but the new results suggest] why humans spend at least half their sleeping hours in Stage 2, non-REM...'