HAVE you ever had one of those dreams where you suddenly realise you haven’t done any revision for an important exam?

Or what about the one where you are being chased and no matter how hard you run you just can’t escape?

There are others, of course, such as the classic one where you are naked in public and then are the ones that are just weird – those inexplicable dreams which feature people you haven’t seen, or even thought about for years, returning out of the blue to your subconscious probably doing something odd like riding a frightening pantomime horse while throwing cabbages at David Cameron.

Dreaming, however, is a serious business and those often bewildering images which play through your mind while your are asleep are all there for a reason.

Marcia Malory, a former New Yorker who has made her home here in York, has been studying the meaning of dreams and the reason we dream, and publishing her work for years.

She explained some of the theories on why we dream, what they mean and why they can be so strange.

“The stage of sleep where we are most likely to dream is called REM sleep,” she said.

“It is also called paradoxical sleep because our mind and body are very active but not in the same way as when we are awake.

“There is a theory that dreaming is a way to create a virtual reality where you learn to do things to prepare you for life but without acting them out.

“All mammals and birds have the same type of REM sleep. We can’t tell what they dream because we can’t read their minds but we think it’s evolutionary.”

According to scientific theory, said Marcia, REM sleep plays a part in the regulation of body temperature and also plays a part in reducing stress.

She said: “People with post-traumatic stress disorder don’t have as much REM sleep, but there’s a drug they can get which increases the amount of REM sleep they have and that reduces the bad dreams.”

Nightmares, Marcia explains, are something almost all of us will experience at some time. “A lot have a negative element to them, it’s just about how bad we find it personally.

“There is also another type of bad dream called night terrors, which are different from nightmares and are a different stage of sleep. It’s believed to be when you are in a deep sleep but part of your brain is awake. It happens mostly in children but they tend to grow out of it.”

Night terrors are more intense than nightmares, being described as bringing feelings of dread and can be distressing for parents to witness in children who experience them.

Luckily for most of us, dreams are usually just a strange series of events played out in our minds while asleep. Offering a possible explanation as to why dreams can be so odd, Marcia said one theory is that a lot of your brain is resting, but you are trying to deal with a lot of ideas and so use a lot of symbolism.

“Then there’s another older theory that your brain stem is giving off signals and your brain is trying to deal with them.”

We asked Marcia what people dream about the most and what they meant. She said two of the most common were falling and being chased.

“Dreams about falling are to do with not having control, she said. “But there’s also a theory that it has something to do with a physical sense which happens when you are asleep.

“Being chased perhaps has something to do with something you can’t escape from – you are trying to avoid something. Very often being chased can also seem as though you are paralysed.”

Marcia also said that very common dreams could be just part of evolution and that dreams of trying to escape something may be a hangover from our days of running away from huge, dangerous animals.

Another common and often unpleasant dream, which may also have evolutionary links, features the loss of teeth.

“It’s to do with health,” she said. “When problems with your teeth may have been one of the first visible signs of illness. It’s also to do with image. it could be about growth and evolution too.

“Your baby teeth fall out so the dream could be about some kind of change in your life.”

Image could also play a part in another very common dream – being naked in public.

“If you are really embarrassed in the dream it could be about how you feel about the way people perceive you,” she said. “But clothing in dreams is about what roles you are playing in life.

“If you feel fine about being naked in the dream it could be about those feelings of freedom you want to recapture from when you were a child.”

Marcia Malory publishes articles on dreams and sleep.

Her website can be found at meaningofdreams.org