When it comes to sleep, you go through 5 different stages.
Stage 1
This is the light sleep stage where you drift in and out of sleep. You can be awakened pretty easily in this stage. During this time, if you're awakened, you may only remember very little, if anything at all. Your muscles can still move at this time.
Stage 2
You'll spend about half of your sleep in this stage. Your eye movements stop and your brain waves become slower.
Stage 3
At this point, your brain waves get even slower. These are called delta waves. There are still some faster waves appearing during this time.
Stage 4
At this point, your brain is just producing mainly delta waves. There is no eye movements of muscle activity. At this point, it is difficult to actually wake. This is the point of the deep sleep. Waking during this point will make you feel disoriented and groggy.
Stage 5
This is the final stage where you experience REM sleep. You will experience heavier breathing; irregular and shallow. Your eyes jerk around rapidly and your muscles become temporarily paralyzed. Heart rate and blood pressure increase during this time. Being awakened at this point, you'll often remember dreams.
REM has different periods. The first happens about 60 - 90 minutes after you fall asleep. The complete sleep cycle takes 90 - 120 minutes.
During your entire night, you'll spend most of your time in stages 1, 2 and REM.