Dreaming is the subjective experience of imaginary images, sounds/voices, words, thoughts or sensations during sleep, usually involuntarily. The scientific discipline of dream research is oneirology. Dreaming has been associated with rapid eye movement (REM) sleep, a lighter form of sleep that occurs during the later portion of the sleep cycle, characterized by rapid horizontal eye movements, stimulation of the pons, increased respiratory and heart rate, and temporary paralysis of the body but this association is questioned since it may be that dream recall after REM sleep is common and because dreams are easier recalled after waking from the light REM sleep. It also occurs in other phases of sleep, though dream recall is more difficult. Hypnogogia, which occurs spontaneously during the approach to deep sleep, is thought to be related to dreaming. Dreams are also associated with male erection about as frequently as with REM sleep.
Dreams are full of imagery. This imagery ranges from the normal to the surreal; in fact, dreams often provoke artistic and other forms of inspiration. Forms of dream include the frightening or upsetting nightmare and erotic dreams with sexual images and nocturnal emission.
Most scientists believe that dreams occur in all humans with about equal frequency per amount of sleep. Therefore, if individuals feel that they did not dream or that they only had one dream in any given night, it is because their memory of the dream has faded. This "memory erasure" aspect of the dream state is mostly found when a person naturally awakes via a smooth transition from REM sleep through delta sleep to the awake state. If a person is awoken directly from REM sleep (e.g. by an alarm clock), they are much more likely to remember the dream from that REM cycle (although it is most likely that not all dreams will be remembered because they occur in REM cycles, which are interrupted by periods of delta sleep which in turn have a tendency to cause the memory of previous dreams to fade).
For a long time true dreaming had only been positively confirmed in Humans, but recently there have been research reports supporting a view that dreaming occurs in other animals as well. Animals certainly undergo REM sleep, but their subjective experience is difficult to determine. The animal with the longest average periods of REM sleep is the armadillo. It would appear that mammals are the only, or at least most frequent, dreamers in nature, which is perhaps related to their sleep patterns. Many animals such as frogs probably do not sleep at all (except when in hibernaculum, which is a different kind of state). Some researchers have managed to deter the function of brain mechanism that locks body and limb movements during dreams. With this method it has been discovered that a cat seems to dream mostly about chasing a prey and playing with its prey.
Neurology of dreams:
There are two competing theories as to the neurological cause of
the dreaming experience. The state of REM sleep is known to be produced
by a brain region known as the pons. The activation-synthesis theory
(developed by Hobson and McCarley) states that the brain tries to
interpret random impulses from the pons as sensory input, producing
the vivid hallucinations we know as dreams. Sensory-based input
interpretation is in turn based on past experience. Perhaps this
is the reason why our dreams contain many characters and scenes
from our regular lives. For some people, there are dreams that recur
again and again over many years, sometimes with new additions derived
from new experiences during waking life. However, research by Mark
Solms seems to suggest that dreams are generated in the forebrain,
and that REM sleep and dreaming are two different brain systems.
The debate between these two theories is ongoing.
