N1 is the first stage of sleep, normally lasting only a few minutes. You experience N1 as you are just drifting off to sleep. During this stage, you may experience strange noises, lights, or sensations, which are known as hypnogagic hallucinations. You may also experience random twitches in your skeletal muscles. These are called hypnic jerks. They can wake you as you fall asleep. Both hypogagic hallucination and hypnic jerks are completely harmless, although they can be startling sometimes.
N2 is the second stage of sleep, and is characterized by a total loss of consciousness. You cease to be aware of any of your surroundings as you fall into a deep, restorative sleep.
N3, also known as slow-wave sleep, was previously broken up into N3 and N4. Recently, it was discovered that there was no discernible difference between stages N3 and N4, so they were combined into N3. It is the deepest sleep out of all of the stages, so it is very difficult to wake someone in the N3 stage. Parasomnias like sleepwalking and night terrors typically occur in this stage. Typically you will experience another period of N2 before moving on to the REM sleep.
REM is probably the most important sleep stage for those who are interested in dreams, because this is when we experience dreams. While there have been recordings of dreams during the other sleep stages, generally REM is considered the stage in which we experience our dreams.
In the first few sleep cycles of the night, REM is extremely short--only a few minutes. But as the night goes on, you spend longer periods of time in REM, up to 30 minutes or more. Babies and children spend most of their sleep time in this stage, but as we grow older we spend less time in REM. If you are an adult, REM comprises about 20-25% of your total sleep time.
During REM sleep, your body causes the atonia, or paralysis, of the skeletal muscles. This is a very good thing, because you would not want to be acting out your dreams in real life! It is still unknown exactly how the body triggers this and has been the subject of much study. Sometimes, you may become conscious while your muscles are still paralyzed. This is known as an episode of sleep paralysis.
Sleep paralysis is an episode in which a person is usually transitioning from wake to sleep or sleep to wake and they find that they cannot move. Sleep paralysis is commonly characterized by hallucinations, vibrations, loud ringing or roaring noises in the ears, pressure on the chest or choking sensations, and often fear of impending doom or terror if the person is has no prior knowledge or experience of sleep paralysis.
Physiologically speaking sleep paralysis is caused by atonia of the voluntary muscles due to the REM cycle. An episode of sleep paralysis occurs because you are either entering or exiting REM sleep and you have become conscious during this transition.
Lucid dreamers are most interested in REM sleep because it is known as the stage in which most dreams occur. Some lucid dreamers take advantage of sleep paralysis, or even induce it, in order to enter directly into a lucid dream. This is known as a WILD (Wake Initiated Lucid Dream).
Other lucid dreamers use their knowledge of sleep stages to set alarms for themselves in the later half of the night, so that they can induce a lucid dream more easily and at a time when REM is longest.