Sleep Paralysis may be one of the most frightening, least dangerous things one can experience in sleep. It usually happens just when falling asleep or right at awakening and is the distinct sensation that one can not move or speak. The feeling is caused by a slight mismatch in the brain and body, in that one is awake and the other still "asleep". It usually only remains for a few seconds, but can last minutes, which for the uninformed can be terrifying. The episode is usually ended by a sound or movement in the room that can "break the spell". I have told patients in the past to just concentrate on moving one tiny muscle, like an eyelid or pinky finger. This usually allows full movement to return quickly.
When sleep paralysis occurs at the onset of sleep it is called "hypnopompic sleep paralysis". In this situation the body is relaxing more and more while the mind is kind of surfing between wake and sleep. If the mind wakes up and finds the body far more relaxed than when it last checked it can lead to a startling feeling of being hard to move.
When it happens upon waking in the night or morning it is referred to as "hypnogogic paralysis". This is more likely when waking out of REM or dreaming sleep. During REM sleep it is natural and normal that our big muscles should be completely limp or paralyzed. This keeps us from acting out our dreams all night – which is a good thing! However if we again have that mismatch and the brain wakes up from REM just a fraction before the muscles wake up we will "catch ourselves" in that paralyzed state.
Sleep paralysis happens more often if we're not getting enough sleep or if our schedules are really irregular. A change in time zones can be a trigger as can some medications that alter sleep cycles or timing. It may occur only once in a lifetime or become a recurring experience. It is usually harmless and once one knows what is happening it can be kind of entertaining. It's the only time we ever get to watch ourselves sleeping!
If sleep paralysis is frequent enough to interfere with refreshing sleep it can be addressed by improving sleep habits, catching up on sleep and working on some general stress relief. There are some antidepressant medications that change sleep stages in such a way that sleep paralysis would be less likely, though this would be reserved for a fairly severe case. It can be associated with Narcolepsy, though, so if sleep paralysis is a common occurrence and there is significant daytime sleepiness, an evaluation by a sleep specialist would be advised.