The brain needs sleep to conduct activity not done during the day such as processing information received during the day, resetting you systems and creating new ideas.
The increased levels of melatonin promote the immune system activity. Thus your body obtains protection from viruses and can obtain some anticancer properties. Human growth hormones are increased which fosters optimal body weight.
The sleep cycle begins with falling asleep. For a while the person is in light sleep, then follows a deeper sleep and finally we have R.E.M. sleep. R.E.M. sleep stands for "rapid eye movement" which occurs during the deepest sleep (when the body is as if paralyzed while the eyes move). These cycles take about 90 minutes with the R.E.M. cycle in the last 30 minutes. There are 4-6 cycles a night.
It takes 60" to get to R.E.M. which is needed for rest. If there are too many disturbances and no R.E.M. then there is no restorative sleep.
You can see why it is recommended that you have at least six hours sleep.