What is "Sleep"?
Sleep can be defined as “a normal, reversible and recurring state of reduced responsiveness to external stimulation which is accompanied by complex and predictable changes in physiology. These changes include coordinated, spontaneous, and internally generated brain activity […] and relaxation of musculature.” (Encyclopædia Britannica Online, 2013).
How can you distinguish between sleep stages from the outside?
In order to reliably distinguish sleep from being awake, the electrical activity of the brain, the eyes and the muscles have to be recorded (Dement, 1996). Brain activity can be measured using electroencephalography (EEG), eye movements using electrooculography (EOG) and muscle activity using electromyography (EMG). This procedure is termed polysomnography (PSG), and can include additional physiological parameters such as respiration recording or heart rate recording (Iber & American Academy of Sleep Medicine, 2007).
Using widely accepted rules on these physiological parameters, such as the Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects (Rechtschaffen & Kales, 1968) or the newer American Academy for Sleep Medicine Manual for the Scoring of Sleep and Associated Events (Iber & American Academy of Sleep Medicine, 2007), it is possible to objectively classify not only whether a subject is asleep, but also different sleep stages that are common among humans. Objectively thereby means, that there is no interaction with or help from the subject needed.
Which sleep stages exist?
The most obvious physiological parameter is the occurrence of rapid eye movements (REMs) measurable by EOG, dividing the human sleep into REM sleep and NREM sleep (not REM sleep). According to the scoring framework of the American Academy for Sleep Medicine (Iber & American Academy of Sleep Medicine, 2007), the NREM sleep can further be divided into three different sub-stages:
N1 sleep (also called somnolence or drowsy sleep), which is characterized by predominant brain activity of 8-13 Hz (alpha waves) and 4-7 Hz (theta waves), sudden muscle twitches, hypnic jerks, sometimes hypnagogic hallucinations, and the lose of some muscle tonus;
N2 sleep, which occupies about 45-55% of adult sleep time, characterized by specific brain activity patterns called sleep spindles (activity of 11-16 Hz) and K-complexes, and further decreased muscular activity;
N3 sleep (deep or slow-wave sleep), characterized by predominant brain activity of 0.5-2 Hz (delta waves). Parasomnias such as sleep walking and sleep talking take place in this sleep stage (Pillar & Malhotra, 2002).
REM sleep was discovered in the middle of the last century by Aserinsky and Kleitman (1953). Besides its obvious rapid eye movements, it can be characterized by low chin muscle tone, transient muscle activity, low amplitude, and mixed frequency EEG, and sawtooth-like wave patterns in the EEG (Iber & American Academy of Sleep Medicine, 2007).
REM sleep shows typical properties of deep sleep (e.g., the subject is hard to wake), but also of wakefulness (e.g. similar EEG patterns). Thus, REM sleep is also called paradoxical sleep (Myers, 2004; Dement, 1998).
How are the different sleep stages arranged throughout the night?
In a normal, undisturbed night sleep stages alternate between REM and NREM sleep. One so called cycle of NREM and REM sleep lasts approximately 90 minutes, prolonging towards the end of the night up to 120 minutes. The ratio of REM to NREM sleep varies during the night: At the beginning of the night, NREM and especially N3 sleep dominate, whereas in the last cycles of the night, long REM sleep periods dominate and there is nearly no N3 sleep (Billiard & Santo, 2003). Figure shows an idealized illustration of the nocturnal sleep cycles.