It was found that if volunteers were awakened in REM periods they nearly always remembered dreaming but in the other stages dreams were very few. The first REM period lasts perhaps 10 minutes and is followed by normal stage 2 or stage 3 sleep, but REM sleep returns about 90 minutes later. As the night progresses the length of the REM periods increase to 15 or even 20 minutes and the intervals between them decrease from 90 to perhaps 20 minutes. Altogether there may be half a dozen REM periods each night, with sometimes as much as 2 hours of dreams, and as many as 15 brief awakenings, but we usually have little memory of the awakenings or the dreams. During REM there are actually three independent neural systems which cause muscular paralysis, blockade of sensory input and cortical activation.
Muscle paralysis can be very worrying if one is conscious of it.
Muscle paralysis and sensory inhibition systems have apparently evolved specifically to enhance our ability to dream for significant periods of time without waking. Freud insisted that every dream represented the fulfilment of a wish, and that nightmares represent masochistic wish fulfilment, but this is not accepted nowadays, nor are his ideas that our subconscious minds are entirely dominated by sex. It has been suggested that many of the characters appearing in our dreams are created by our subconscious minds and that dreams are a means of sorting out some of our mental problems. This would explain why we need to dream and why people become very disturbed if their REM sleep is constantly interrupted. It seems that an antagonistic character, or demon etc.