Sleep is generally characterized by a reduction in voluntary body movement, decreased to little reaction to external stimuli, loss of consciousness, a reduction in auditory receptivity, an increased rate of anabolism (the synthesis of cell structures), and a decreased rate of catabolism (the breakdown of cell structures). The capability for arousal from sleep is a protective mechanism and also necessary for health and survival.
Sleep progresses throughout the night in cycles of REM and NREM phases. In humans, these cycles are approximately 90 to 120 minutes long and each phase may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see Sleep deprivation). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.
In REM sleep, the brain is active and the body inactive, and this is when most dreaming episodes occur. REM sleep is characterized by an electroencephalography (EEG) that has low voltage and mixed frequencies, similar in appearance to the awake EEG. During REM sleep the sympathetic nervous system is active, but there is a loss of skeletal muscle tone and our muscles are paralyzed so that we don’t act out our dreams.
In NREM sleep, the body is active, while the brain is relatively inactive compared to REM sleep, and there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered ‚light sleep‘, and 3 and 4 ‚deep sleep‘. They are differentiated solely using EEG and unlike during REM sleep which is characterized by rapid eye movements and relative absence of muscle tone, during NREM sleep limb movements are quite frequent and sleep walking (parasomnia) can occur in non-REM sleep.
EEG is the most commonly used method to detect REM/NREM Stages, complemented by additional physiological parameters like ECG, EMG, EOG, respiration or heart rate. Practical problems such as avoiding electrode detachment during sleep are minimized with Brain Products comfortable electrode caps where flat electrodes are inserted directly into an infracerebral textile fabric cap.