There are basically 5 stages of sleep in a typical 8 hour night.  You will experience four or five 90 minute sleep cycles during this time. 

Stage 1 Drowsiness:  This stages can last anywhere from 10 seconds to 10 minutes.  Your breathing becomes more shallow as you transition from waking to falling sleep.  You can respond fairly quickly to disturbances as you are still aware of what is going on around you.

Stage 2 Light Sleep:  This is the beginning of sleep and you are no longer aware of what is going on around you.  Your eye movements stop, blood pressure drops, and muscles relax more. 

Stage 3 & 4 Deep Sleep:  The only difference in these two stages is that 4 is slightly deeper than 3.  During these two stages your body is actually very busy working.  Your body repairs tissues, regenerates, and develops muscle and bone.  Your immune system is boosted, and energy is restored from the days use.  This deep sleep can last from 40-60 minutes.  If you are woken up now it will take about 15-20 seconds to realize where you are. 

Stage 5 REM:  Rapid Eye Movement is just as it sounds.  Your eyes move rapidly in different directions while your brain is working at preserving memory.  You are dreaming and should remember these dreams upon waking.  The first REM period only lasts about 9 minutes.  But as the night progresses, each REM period doubles in length.

Although the experts do not agree on the exact amount sleep we need, they all agree there are many benefits and various methods to get plenty of sleep.