The dream is defined as "a state of brain and body controlled by the diencephalic system and the brainstem, characterized by regular and reversible loss of consciousness, reduced sensory and motor functions of the brain in its interaction with the environment and which is regulated internally by homeostatic and circadian mechanisms and restorative functions, which can not be replaced by rest or sleep without any food, drink or drug. Sleep is as essential as food and water. " Michael S. Aldrich, author in the research on the subject, define it as a biological and behavioral state, a mysterious phenomenon necessary for the proper physical and mental functioning of all human beings. There is evidence, that all mammals sleep, although it has not yet been determined why. In the case of humans, we spend approximately one third of our lives and many of the circumstances that occur during sleep are still unknown.
Under normal circumstances, a pattern of sleep cycles is organized sequentially repeated throughout a day of sleep. Cycles in turn are divided into different stages based on the level of depth and physiological characteristics of each one of them. Each stage or phase change corresponds to a change in the neurophysiological pattern specific. To make this distinction primarily it has to be conformed to three factors, muscle tone, brain waves and eye muscle movements. These three indicators and the changes they undergo, allow establishing the five stages of sleep. This information is obtained through polysomnographic recordings, studies carried out overnight in the sleep units and recording of neurophysiological variables of the subject while asleep.
Sleep is divided into two separate states or two types of sleep. The first state is called NREM sleep or slow wave sleep and the second REM sleep or rapid eye movement sleep or REM sleep. In turn, these stages are divided into phases depending on the characteristics of brain waves reflected in the electroencephalogram (EEG). The first four NREM sleep stages correspond to and include the initial light sleep to deep sleep stage. The last stage is REM sleep which completes the circle and presents more obvious differences in the EEG compared with the NREM sleep stages.
|Stages of sleep and physiological activity|
|Vigil||NREM sleep||REM sleep|
|EEG||Fast, low voltage||Slow (slow waves), high voltage||Fast, low voltage|
|Eye fast movements||Fast||Slow, infrequent||Fast|
|Muscle tone||High||Moderate||Non-existent (with the exception of eye muscles and diaphragm)|
REM and NREM states alternate in cycles of 90-110 minutes. A healthy adult completes several cycles during a night that included the four stages of NREM sleep and REM sleep. Each complete cycle lasts approximately 90 to 110 minutes and is repeated in sequential form.
During one night each subject experiences five or six complete cycles. For making sleep restorative, cycles must repeat this pattern without disturbances, that could fragment it. Also, sleep cycles have a different composition depending on the time of the night. Throughout this period, there is an uneven distribution of REM and slow wave stages (stages III-IV): during the first half of the night abounds slow-wave sleep while REM sleep is low (the first period usually lasts 5 minutes). By contrast, in the second half of the night the distribution is reversed: abundant REM sleep (stages progressively longer, the last could reach at the 25-30th minute), and virtually disappears almost entirely slow-wave sleep.
The structure of sleep is not always the same in all subjects and varies substantially depending on individual circumstances and individual differences that influence the onset and duration of each phase. To explain this evolution using the histogram or hipnograma. Through these graphs the temporal content of the different phases of sleep are represented and its distribution throughout the evening by taking as a reference a dream of one day of an adult of eight hours.