Sleep Cycles

One sleep cycle comprises of four stages and last for about 90-120  minutes. Dreams can occur in any of the four stages of sleep but the most vivid and memorable dreams occur in the last stage of sleep (also commonly referred to as REM sleep). The sleep cycle repeats itself about an average of four to five times per night, but may repeat as many as seven times. Thus, you can see how a person can have several different dreams in one night. Most people, however, only remember dreams that occur closer toward the morning when they are about  to get up. But just because you can't remember those dreams does not mean that  they never happened. Some people swear on the fact that they simply do not dream when in reality, they just don't remember their dreams.

Stage 1: In this stage of sleep, your eyes move back and forth erratically. Often called REM sleep, this stage occurs at about 90-100 minutes after the onset of sleep.  Your blood pressure rises and heart rate and respiration speeds up and becomes erratic. Your voluntary muscle are paralyzed. This stage may also be referred to as delta sleep and is the most restorative part of sleep. This is also where the majority of your dreaming occurs.

Stage 2: You are entering into light sleep. This stage is characterized by Non-rapid eye movements (NREM), muscle relaxation and slowed heart rate.  The body is preparing to enter into deep sleep.

Stage 3 and 4: Also characterized by NREM, these two stages involves periods of deep sleep with Stage 4 being more intense than Stage 3.   Your body temperature drops and muscles relaxes. You are completely asleep.

These stages repeat themselves throughout a night's sleep.

From dreammoods.com