We've all been there -- dead asleep, caught up in the middle of a cinematic dream that feels so real you think you've actually experienced it, even after waking. Maybe it was a nightmare that left you in a cold sweat, heart pounding. Or if you're lucky, it's a liaison with your favorite movie star. Sigmund Freud believed that dreams are a window into our unconscious, and some studies indicate that he may have been onto something. For example, in one study, amnesiacs reported dreaming about activities that the scientists knew the patients had participated in before they'd gone to sleep -- even though the amnesiacs had no memory of those activities, outside of dreaming about them. This validates Freud's theory to a certain degree, but there are hundreds of competing theories about what dreams are and what their purpose is.

So what are dreams? Strictly speaking, dreams are images and imagery, thoughts, sounds and voices, and subjective sensations experienced when we sleep. This can include people you know, people you've never met, places you've been, and places you've never even heard of. Sometimes they're as mundane as recalling events that happened earlier in the day. They can also be your deepest and darkest fears and secrets, and most private fantasies. There's no limit to what the mind can experience during a dream and really no rhyme or reason to what you end up dreaming about. Stresses in waking life can manifest in dreams plainly or be cleverly disguised with imagery. For instance, a dream about a grizzly bear chasing you through your house could be the stress you feel about the relationship with a friend. A dream about being stuck inside of a room with no door might echo your feelings about a dead-end job.

Dreams are most abundant and best remembered during the R.E.M. stage of sleep. This is the deepest stage of the sleep cycle, when your eyes are moving rapidly, your heart rate and breathing become inconsistent, and paralysis of your skeletal muscles occurs. This last part may sound pretty frightening, but it's actually a safeguard that keeps us from acting out our dreams physically. R.E.M. sleep makes up about 20 to 25 percent of the sleep cycle in adults in short increments, first for only a few minutes at a time, but growing increasingly longer as the sleep cycle progresses. Some stages of R.E.M. sleep can last for as long as 30 minutes at a time. Since long, detailed dreams can sometimes only last a couple of minutes, these long R.E.M. stages can be rich with dream activity.

It's believed that we all dream, even though not everyone remembers their dreams with the same frequency, or at all, in some cases. Consider that we may dream as much as six to eight hours per night and you have a clue as to how many dreams are left behind. Waking up during a R.E.M. cycle will make it more likely that you'll remember your dreams, but there's no hard and fast rule regarding dream memory. Scientists know waking experiences have an impact on our dreams, but they aren't sure how much or if it's consistent among humans. In one study, participants wore red goggles before going to sleep and reported seeing more red images in their dreams than those who didn't wear them.

Another theory posits that dreams are our brain's attempt to make sense of what would otherwise be meaningless stimuli, random messages created from the arousal of the posterior segment of the brain. Others think dreams are merely the brain firing signals as it organizes the previous day's thoughts and experiences that may end up as memory. The problem with any theory about dreams is that we can't really prove or disprove any of them, and they aren't necessarily mutually exclusive. If dreams are psychologically significant, they can still be the result of random brain stimuli. Dreams are most likely a combination of theories.

There's still so much we don't know about the brain and how it operates, waking or sleeping, that we may never be able to pinpoint exactly what dreams are, and how they're meaningful or necessary for humans. But that doesn't stop science from trying. Dream studies are always among the most popular in universities and research facilities, which probably has something to do with their ubiquitous nature.