IMPORTANT NOTE: This is not a medical site. You should always discuss medical matters with your doctor.
Why Do We Sleep?Sleep is a near universal trait. All mammals, birds and reptiles sleep, even if sleeping is in fact dangerous (leaving them prone to predators). From a human perspective - with fewer night time predators to worry about - sleep is simply "wasted" time that could be "better" spent doing other things.
The amount of sleep humans need varies from one person to another but averages eight hours a night. That's almost a third of our life. Why do we need sleep? Why do we need so much "downtime"?
The Benefits of SleepSleep has both physical and mental benefits. Physically it is the time when the body can renew its energy store. Experiments on rats have shown that lack of sleep supresses the immune system.
Sleep is also the time when the body does its best "repair work", fixing ailments and fighting invaders. Experiments have shown that growth hormones are produced during sleep which assist in tissue growth and repair.
You've probably noticed that when you're sick or hurt you feel more tired more often. This is probably the body's natural mechanism to make you sleep and allow it to repair itself.
However sleep is not simply about "rest". People who spend the day idle still need to sleep. If you are too fatigued then it can actually be harder to get to sleep.
The mental effects of sleep are less well established than the physical ones. There are numerous psychological and analytical theories as to what dreams mean - but what are they for?
It's interesting to note that during sleep the brain is not idle - it's actually very active. Yes, sleep probably helps us to rebuild our mental energy, but it's not as simple as just resting.
One possibility is a purely physical one: sleep allows certain nerve cells to rest. Even though the brain as a whole is active, especially in REM sleep, certain cells such as those involved with serotonin transmission are switched off. It's possible that such neurons can become "overused" during the day and need sleep to recover.
Some researchers believe that dreams reflect the mind sorting out the business of the day. It's like "sticky notes" on a computer monitor. During the day too many things happen for us to analyse them all, so we scribble them on mental "sticky notes". During sleep, the brain goes through these "stickies" filing them and marking some for action. Dreams are a side-effect of this process.