Sleep is a state where the body is in unconsciousness and it gives more response to the internal stimulus rather than the external stimulus. Sleep cycle may be classified into 5 stages that are stage1,stage2,stage3,stage4,REM that are classified by Rechtschaffen and Kales and the other cycle that is considered by the American Academy of Sleep Medicine that are n1,n2,n3 (in case of n3 stages s3 and s4 are merged),w,r are said as stages wake. Stage 1: It is the lightest stage of NREM sleep. Occasionally people may experience hypnic jerks or abrupt muscle spasms and may even experience sensation of falling while drifting in and out of Stage 1. Stage2: Almost 50% of the time spent asleep over the course of the night is spent in stage two. Stage two is also a non-REM phase and is one of the lighter stages of sleep. Even though it is a light stage, the heart rate begins to slow and the core body temperature decreases. Stage3&4: Stages three and four are characterized as the deep stages of sleep and are often the hardest to wake up from. Stages three and four are often grouped together because they are the periods of slow wave sleep (SWS). Slow wave sleep is a NREM phase of sleep. It is called slow wave sleep because the brain waves slow to what are known as delta waves.