There are two main stages of sleep: REM sleep and non-REM sleep. REM stands for "rapid eye movement." The body starts with non-REM sleep and then switches to a short period of REM sleep before the cycle starts over. Each of these stages have multiple levels of sleep.
During REM sleep, the eyes move quickly in a number of directions; this doesn't happen during non-REM sleep. Dreams are most likely to happen during REM sleep.
There are three main stages of non-REM sleep. The first stage is when the body first falls asleep but the person is easy to wake. This stage lasts between five and 10 minutes. The second stage is light sleep. During this stage, a person's body temperature drops and heart rate slows. This happens as the body prepares for the final stage of non-REM sleep, which is deep sleep. During this level of sleep, it is hard to wake a person. If a person is woken up from this level of non-REM sleep, he will be disoriented for a few minutes upon waking.
REM sleep kicks in around 90 minutes after falling asleep. The first stage of REM only lasts for about 10 minutes. Every stage of REM after that continues to get longer, with the final stage of REM lasting about an hour. During REM, the breathing and heart rate of a person quickens.
Most of the body's repair functions, such as regrowing tissues, strengthening the immune system and building bones, happen during non-REM sleep. As a person ages, he is more likely to fall into deep sleep.