Sleep is defined as a state of unconsciousness in which it is possible to be aroused from this situation by a specific stimilus. There are two known types of sleep patterns. These are slow wave brain sleep and rapid eye movement sleep. Slow wave sleep is so called because the sleep brain waves are usually slow.
Rapid eye movement sleep or briefly REM sleep is so called because of the rapid eye movement that accompanies this type of sleeping. REM sleep happens during sleeping in a periodic manner and constitutes around 25% of the whole sleep time.
Dreams occur during REM sleep although they can occur during slow wave sleep. Nightmares occur especially during slow wave sleeping. In slow wave sleep the manner of sleeping is considered calmful. Associated with this type of sleeping is a decrease in blood pressure of the sleeping individual in the amount between 10% to 30%.
In addition there is respiratory rate decrease and metabolic rate decrease as well observed. Although there is dreaming in both types of sleep patterns, REM sleep dreaming is often remembered after waking up. On the other hand, slow wave dreaming is usually accompanied with forgetfulness of the dreams that occur during this type of sleep.
This is the main difference between the two types of sleeping patterns. During slow wave sleep dreaming the process of remembering is not consolidated into memory and thus it is not remembered after waking up as occurs with REM sleep.
REM sleep as was mentioned earlier is associated with rapid eye movement and dreaming. Beside these two characteristics, several other patterns that are associated with this type of sleep are observed. These characteristics include difficulty to arouse the sleeping person from REM sleep. This difficulty is more than the difficulty to arouse him from slow wave sleep. This is so usually even though usually the sleeping person awakes from REM sleep.
During REM sleep heart rate becomes irregular in addition to irregular respiration patterns. The brain is active during REM sleep in contrast to slow wave sleep . The metabolism of the brain can increase by the amount of 20% of normal metabolic rate. REM sleep is sometimes called paradoxical sleep because in this type of sleep the brain is active although the person is in a state of sleeping.
The brain areas which are responsible or are suspected to trigger the sleeping state include the raphe nuclei in the brainstem. The neurons of the raphe nuclei secrete the neurotransmitter serotonin. It is therefore suspected that the neurotransmitter serotonin secretion is involved in the physiology of sleep and in particular in the induction of sleep.
In addition, it is known that certain areas of the hypothalamus can induce a state of sleeping upon stimulation. Besides the role of serotonin in inducing sleep other compounds were found to induce a state of sleeping when injected into the ventricular system of the brain. One of these compouds is called muramyl peptide.