Some info on Wikipedia, just for reference:

Sleep paralysis is a condition characterized by temporary paralysis of the body shortly after waking up (known as hypnopompic paralysis) or, less often, shortly before falling asleep (known as hypnagogic paralysis).

Physiologically, it is closely related to the paralysis that occurs as a natural part of REM (rapid eye movement) sleep, which is known as REM atonia. Sleep paralysis occurs when the brain awakes from a REM state, but the bodily paralysis persists. This leaves the person fully aware, but unable to move. In addition, the state may be accompanied by hypnagogic hallucinations.

More often than not, sleep paralysis is believed by the person affected by it to be no more than a dream. This explains many dream recountings which describe the person lying frozen and unable to move. The hallucinatory element to sleep paralysis makes it even more likely that someone will interpret the experience as a dream, since completely fanciful objects may appear in the room alongside one's normal vision.

Symptoms: The primary symptom of sleep paralysis is partial or complete skeletal muscle paralysis during the hypnopompic or hypnagogic states. In other words, it is the sense of being aware that one is unable to move or speak while falling asleep or waking up. Sleep paralysis may also be accompanied by hypnagogic hallucinations. These hallucinations can be auditory, tactile, and/or visual. If a polysomnograph is taken, at least one of the following will be shown: skeletal muscle tone suppression, REM sleep at sleep onset, or dissociated REM sleep. The paralysis can persist anywhere from a few seconds to a few minutes before the person is able to either return to REM sleep or to become fully awake.

Possible Causes: Sleep paralysis occurs during REM sleep in order to prevent the body from manifesting movements made in the subject's dreams. Little is known about the physiology of sleep paralysis. However, some have suggested that it may be linked to post-synaptic inhibition of motor neurons in the pons region of the brain. In particular, low levels of melatonin may stop the depolarization current in the nerves, which prevents the stimulation of the muscles, to prevent the body from enacting the dreamt activity (e.g. preventing a sleeper from flailing his legs when dreaming about running).

Many people who commonly enter sleep paralysis also suffer from narcolepsy. However, various studies suggest that many or most people will experience sleep paralysis at least once or twice in their lives.

Some reports read that various factors increase the likelihood of both paralysis and hallucinations. These include:

* Sleeping in a supine position (facing upwards)
* Irregular sleeping schedules; naps, sleeping in, sleep deprivation
* Increased stress
* Sudden environmental/lifestyle changes
* A lucid dream that immediately precedes the episode. Also conscious induction of sleep paralysis is a common technique to enter a state of lucid dreams, also known as WILD.
* Artificial sleeping aids, ADD medications and/or antihistamines