Dreams are usually story-like and narrative, and they often appear to us (rightly or wrongly) to connote something meaningful about our lives. They range in subject matter from the normal and ordinary to the illogical, bizarre and downright surreal. In fact, most dreams are surprisingly ordinary and unexciting, usually based on recent life events, but we are more likely to remember the more flamboyant, strange or alarming ones. The events in a dream still feel totally realistic and logical to us, however fantastical they may be.
Sexual fantasies can and do appear in dreams, but not to anything like the extent popularly thought (between 8%-10% according to some estimates, and more common in young to mid-teens). By some estimates, at least 40%, and perhaps as much as 75%, of normal dream content is negative in nature, although only a small minority of these would be characterized as nightmares (see the separate section on Nightmares). Whatever the subject matter, though, dreams are always egocentric, involving ourselves as a principal character. Despite this, we have no control over the story-line of a dream, and generally we are not even aware that we are dreaming (except in the case of lucid dreams - see the section on Other Kinds of Dreams).
Most people dream in colour, but some people do dream in black and white. Interestingly, much fewer people report dreaming in black and white now than did 50 years ago (when television was black and white), and most of those that do still claim to dream in black and white are of an age to remember black and white television. This has led some to conclude that media exposure as a child influences, or perhaps in some way reconstructs, our dreams (or at least our memories of dreams). However, it is impossible to objectively verify such reports, and it is difficult to draw any firm conclusions about them.
Dreams can occur in almost all stages of sleep, but they are most common during REM sleep, particularly towards the end of the sleep period, and the dreams experienced during this sleep stage tend to be more vivid, detailed, memorable and often bizarre. During REM sleep, there is an almost complete loss of muscle tone and skeletal muscle activity (known as atonia), and it has been speculated that this paralysis may be a built-in mechanism to protect the dreamer from any injuries that might occur if they were to physically act out the vivid, and often violent, content of REM dreams. This may also account for the commonly-experienced dreams in which we feel unable to move or can only move sluggishly in response to a threat. Studies during the 1970s showed that, where REM sleep is denied to sleepers, dreams assert themselves anyway, either during other sleep stages or even intruding themselves into daytime wakefulness, suggesting that the urge to dream is so strong that the brain seeks to compensate for its loss.
There is also a phenomenon called dream incorporation, in which outside stimuli can be incorporated into the narrative of dream imagery. William C. Dement and others did many experiments on this subject back in the 1950s and 1960s. Examples might be a dream about braving Arctic conditions caused by the bedcovers slipping off, the incorporation of particular friends into a dream in response to the prompting of the friends' names, the incorporation of a phone or alarm clock's ringing into a dream, etc . Often the stimuli are changed or twisted in some way, but still recognizeable. Some scientists believe that this represents the body's way of protecting and extending REM sleep despite external interferences.
Everyone dreams every night, from early childhood until the day we die, regardless of whether or not they can remember those dreams. Memories of dreams are very unstable, and tend to disappear completely within a few minutes of waking, unless we make a deliberate attempt to remember them, or write them down. In experiments, up to 80% of adult sleepers woken during REM sleep can remember at least some of their dreams. Light sleepers tend to be able to remember their dreams more easily than deep sleepers. Some people can never remember their dreams, regardless of when they were woken.
Although it is known that babies spend long hours of the day and night in REM sleep (and even more in utero), it is still not known exactly when they actually start to dream. Young children do definitely dream, although only about 20% of 7-year olds report dreams when woken from REM sleep, as compared to around 80% of adults, even though they spend substantially more time in REM sleep than adults.
The dreams of young children usually tend to be quite static and plain, featuring few characters and social interactions, and little in the way of strong emotions. Many childhood dreams are "primal" or atavistic in nature, similar to the kinds of dreams mankind may have experienced millennia ago. At first, they may not even include the dreamer as a major participant, although by the age of about seven that starts to change. It is hypothesized that it is the development of mental imagery and visuo-spatial skills, rather than verbal or memory capabilities, that allows children to begin to experience complex and personalized dreams.
Blind people also dream, just like everyone else, but, if they were born blind and have therefore never seen the world around them, the visual aspect of dreams tends to be lost. Their dreams are largely made up of sounds, tactile sensations and smells, senses which are often very highly developed in blind people.
Other mammals and, to a lesser degree, birds also appear to dream, in that they go through the same physiological processes as humans (REM brain-wave patterns and eye movements, twitches, etc). It is thought, however, that reptiles, fish and insects, with their simpler brain processes, do not dream.