Sleep: Encyclopedia II - Sleep - Sleep physiology

Sleep - Sleep physiology

Sleep - Methodology

Before advances in the fields of neurology, neuroscience, electronics and genetics were made, scientists studied the behavioral characteristics of sleep, such as its pattern, depth, and varying frequency. In more recent times, the electrical impulses generated by the brain are recorded using a device called an electroencephalograph (EEG), and individual genes relating to sleep-related brain function, such as the circadian rhythm, have been isolated. Molecular biology, medical science and epidemiology all play an important role in modern studies of sleep.

Sleep is often defined using specific criteria relating to EEG data. All mammals and birds fulfill the criteria for sleep based on EEG recordings. In animals where EEG data is not readily available, or their small size precludes recording an EEG, behavioral and gene specific data are utilized for sleep studies.

Sleep - Sleep regulation

The cycle of sleep and wakefulness is regulated by the brain stem, external stimuli, and various hormones produced by the hypothalamus. Some neurohormones and neurotransmitters are highly correlated with sleep and wake states. For example, melatonin levels are highest during the night, and this hormone appears to promote sleep. Adenosine, a nucleoside involved in generating energy for biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep. Researchers believe that its accumulation during the day encourages sleep. The stimulant properties of caffeine are attributed to its negating the effects of adenosine.

The suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role in the regulation of circadian rhythms. The SCN is influenced by external light and also generates its own rhythm in isolation. In the presence of light it sends messages to the pineal gland that instruct it to cease secreting melatonin.

Thus, three processes, each influenced by hormonal, neurological, and environmental factors, underlie sleep regulation:

• A homeostatic process determined by prior sleep and wakefulness, determining "sleep need".
• A circadian process determining periods of high and low sleep propensity, and high and low rapid eye movement (REM) sleep propensity.
• An ultradian process

The interrelationships and relative importance of each process and system remain uncertain.

Sleep - Stages of sleep

Studies of human sleep have established five well-defined stages, according to electroencephalographic (EEG) recordings and polysomnography:

• Non REM sleep (NREM), which accounts for 75-80% of total sleep time:
  • Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. The stage is sometimes referred to as somnolence, or "drowsy sleep". It appears at sleep onset (as it is mostly a transition state into Stage 2), and can be associated with so-called hypnagogic hallucinations. In this period, the subject loses some muscle tone, and conscious awareness of the external environment: Stage 1 can be thought of as a gateway state between wake and sleep.
  • Stage 2, with "sleep spindles" (12–16 Hz) and "K-complexes". The EMG lowers, and conscious awareness of the external environment all but disappears. This occupies 45-55% of total sleep.
  • Stage 3, with delta waves, also called delta rhythms (1–2 Hz) is considered part of SWS and functions primarily as a transition into stage four. Overall it occupies 3-8% of total sleep time.
  • Stage 4 is true delta sleep. It predominates the first third of the night and accounts for 10-15% of total sleep time. This is often described as the deepest stage of sleep; it is exceedingly difficult to wake a subject in this state. This is the stage in which night terrors and sleepwalking occur.

• Stage 5, or Rapid eye movement (REM) sleep, associated with dreaming, especially bizarre, visual, and seemingly random dreams. REM sleep is predominant in the final third of a sleep period, its timing linked to circadian rhythm and body temperature. The EEG in this period is aroused and looks similar to stage 1, and sometimes includes beta waves.

Sleep proceeds in cycles of NREM and REM phases. In humans, the cycle of REM and NREM is approximately 90 minutes. Each stage may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see sleep deprivation below). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.

Each sleep stage is not necessarily uniform. Within a given stage, a cyclical alternating pattern may be observed.

Adapted from the Wikipedia article "Sleep physiology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki