A person who gets just a few hours of sleep per night can experience many of the same problems over time. Two other things are known to happen during sleep. Growth hormone in children is secreted during sleep, and chemicals important to the immune system are secreted during sleep. You can become more prone to disease if you don't get enough sleep, and a child's growth can be stunted by sleep deprivation. No one really knows.
Sleep gives the brain a chance to organize and archive memories.
Dreams are thought by some to be part of the process. Sleep lowers our energy consumption, so we need three meals a day rather than 4 or 5. According to this article , sleep may be a way of recharging the brain, using adenosine as a signal that the brain needs to rest. Since adenosine secretion reflects brain cell activity, rising concentrations of this chemical may be how the organ gauges that it has been burning up its energy reserves and needs to shut down for a while. Adenosine levels in the brain rise during wakefulness and decline during sleep.