The different stages of sleep are as follows:
Non-Rapid-Eye-Movement (NREM) Sleep
When you are in stage one, your EEG shows theta waves. Your body is relaxed, and you are drowsy; your body temperature drops. During this period, you are not officially sleeping. The average amount of time spent in this stage is five minutes.
This is considered the first stage of true sleep. Your EEG shows sleep spindles and K-complexes. Your heart rate and breathing slow down. The average time spent in this stage is 10 to 25 minutes initially, but you continue to cycle back to this stage intermittently throughout your sleep. If all you achieve is stage two sleep, you would wake up feeling that you had slept, but you would not feel completely refreshed.
Stages Three and Four
Your EEG shows an increase in delta waves, depending on whether you are in stage three or four. These are your deepest stages of sleep, when your heart rate, blood pressure, and respiratory rate are at their lowest. If you are not able to get enough of these stages of sleep one night, your body will spend more time in these deeper stages the next time you fall asleep, in order to catch up.
Rapid-Eye-Movement (REM) Sleep
During rapid-eye-movement (REM) sleep, your brain is very active, often dreaming, but your body is nearly paralyzed.
Although the amount of time a person spends in the deeper NREM stages of sleep decreases with age, the time an adult spends in REM sleep is fairly constant, at about 20 to 25 percent of the total sleep time throughout life. Actually, newborns spend 50 percent of their sleep time in REM sleep, but the amount drops to 20 to 25 percent by young adulthood. During REM sleep, your body temperature, heart rate, blood pressure, and respiratory rate all increase. It is in this phase that "nocturnal erections" (both penile and clitoral) occur. Generally, the brain passes through REM sleep three to five times a night, about every 90 minutes. At first, the amount of time in REM is only a few minutes, but it increases with every cycle, until the duration is about 30 minutes.