Sleeping is an activity that involves being in a resting state in which any conscious activity and voluntary movement are completely suspended, being a periodic and reversible process, unlike a coma, general anesthesia or hibernation. Speaking of sleep, it is inevitable to talk about dreams. Dreams are reactions to random nervous system stimuli, which the brain "interprets" as bizarre images and other sensory hallucinations (Hobson, 2000)
The human life is developed in cycles. The gestation, that of childhood, adolescence, adulthood, maturity, old age and death. Also, while sleeping there are cycles and changes along its course. These cycles are related to changes in brain electrical activity and we don't dream in all of them. We cycle through five sleep stages in about 90 minutes (Rosenzweig, 1992). These cycles are repeated 4-5 times throughout the night. The five sleep stages are:
Stage 1. Numbness, transition state between sleep and wakefulness. This period can produce micro-sleeps and hallucinations. The electroencephalogram shows brain activity slower than in the waking state. Stage 2. Light Sleep (in which we spend the most time) follows about 20 minutes later. Lowers the heart rate and breathing. During this stage the brain calm periods alternate with other activities. Stage 3. Transitional stage to deep sleep, takes a few minutes. Stage 4. Delta wave sleep. It's a slow sleep in which both, the breathing rate and brain waves, are slow. It's very hard to wake up in this stage, dreams do not usually occur. Then we go back to stage 2, is at this moment when we enter a new physiological state called REM sleep, and what characterizes this stage are rapid eye movements; REM sleep is a recurring sleep stage during which vivid dreams commonly occur. Also know as paradoxical sleep, because the muscles are relaxed (except for minor twitches) but other body systems are active (Myers, 2010).