In our waking hours we are constantly assaulted by outside forces that seek to destroy us: predators, parasites, microbes, poisons, and even fellow humans. Our bodies respond with formidable defenses, though at considerable cost. As we fight for survival, we overuse and damage our muscles; we exhaust our neurons, expending their biochemical reserves while at the same time building up toxins; we struggle to cope with a relentless onslaught of information that we feel, rightly or wrongly, must be assimilated to ensure our survival; we fight pathogens of various sorts that attempt to parasitize our blood or eat our skin. Our own species members are if anything even more dangerous: enemies threaten our status, income, and even our very lives. By evening we crave time to gather our thoughts and make sense of our experiences; we need time to repair the daily ravages that have diminished our bodies. We need an opportunity to rest and regroup. Sleep is that opportunity. Without it we would, quite literally, die. It is a daily chance to reverse all the insults our physical and mental being has suffered during the day and to assemble our resources to face another day.