What is Sleep

Sleep is the regular state of natural unconsciousness observed in
all mammals, birds, and fish. It is heavily influenced by circadian
rhythms and by hormonal and environmental factors as well. Sleep
appears to perform a restorative function for the brain and body,
as evidenced by the myriad symptoms that result when an
individual is deprived of sleep.

The function of sleep in health and in disease is being increasingly
studied in specialized sleep laboratories throughout the world.
Not only insomnia, but also more recently elucidated sleep
disorders such as sleep apnea and narcolepsy are evaluated in
such facilities.

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, are
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 EEG, behavioral and gene
specific data are utilized for sleep studies.

Sleep regulation
The cycle of sleep and wakefulness is regulated by the brain stem,
external stimuli, and by various hormones produced by the
hypothalamus. Certain 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 generates its own rhythm in
isolation. In the presence of light, it sends messages to the pineal
gland that instruct it to cease secreting melanin.

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 REM sleep propensity.

¡¤ An ultradian process.

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

Memory is highly dependent on sleep. REM sleep appears to help
with the consolidation of spatial and procedural memory, while
SWS helps with the consolidation of declarative memories. When
experimental subjects are given academic material to learn,
especially if it involves organized, systematic thought, their
retention is markedly increased after a night's sleep. Mere wrote
memorization is retained similarly well without an intervening
period of sleep.

REM sleep (or Active Sleep) seems to be particularly important to
the developing organism. Studies investigating the effects of
Active Sleep deprivation have shown that deprivation early in life
can result in behavioral problems, permanent sleep disruption,
decreased brain mass (Mirmiran et al. 1983), and an abnormal
amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).

According to the Ontogenetic Hypothesis of REM sleep, the activity
occurring during neonatal REM sleep is necessary for proper central
nervous system development (Marks et al. 1995).

Given sleep's heterogeneous nature, it is difficult to describe a
single "function" of sleep. Based on current knowledge, it is
apparent that it has many functions.