A series of activity takes place in your brain when you go to sleep. Two basic categories of sleep exist namely REM Rapid Eye Movement Sleep and NREM Non-rapid Eye Movement Sleep (has four stages). When one gets to sleep his body wavers between REM and NREM sleep several times. On an average, a healthy individual will begin sleeping cycle in non-REM sleep which is followed by REM sleep (exist for short duration). It is this stage when one gets dreams.
REM sleep is Rapid Eye Movement sleep and during the stage of NREM the body builds up torn tissues and builds up muscles and bone to strengthen the immune system. As one grows old, quantity of sleep gradually reduces for many people, but studies indicate that it is not true. One can get quality sleep even at old age. (Without taking medications)
REM Sleep :
REM sleep starts within an hour of your sleeping time. It is interesting to know that the brainwave patterns are exactly the same when you are fully awake. In other words, during initial phase of REM your brain will be awake. Your heartbeat and respiration rate will be quicker than normal during this stage of sleep and most importantly your eyes will make rapid movements in different directions during REM sleep.
One will get lot of dreams during REM sleep which is due to responsive brain activity. Slowly major voluntary muscles will become paralyzed at this stage. REM is a stage when the brain is in the state of total excitement without any muscle movement. Hence this stage is called as paradoxical sleep. A healthy adult will have only 20% of their sleep at REM pattern and it is slightly more in adolescents (30-35%) and for infants it is 50% of their total sleep duration they would be spending in REM sleep.
Healthy Sleep :
Many people will have the question about the quantity of sleep one should have. There is no concrete rule that you should sleep for fixed hours each day, provided you are active and healthy next day. Need for sleep varies from one person to another and it also depends on various factors.