Page 347 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Sleep–Wake Cycle, Circadian Rhythms The daily sleep–wake cycle and other circadian
rhythms (diurnal rhythms) are controlled by
Various stages of sleep can be identified in the endogenous rhythm generators. The central bio-
EEG (! p. 333 D). When a normal person who logical clock (oscillator) that times these
is awake, relaxed and has the eyes closed (α processes is located in the suprachiasmatic nu-
waves) starts to fall asleep, the level of con- cleus (SCN) of the hypothalamus (! A). The en-
sciousness first descends to sleep phase A dogenous circadian rhythm occurs in cycles of
roughly 24–25 hours, but is unadulterated
(dozing), where only a few isolated α waves
Central Nervous System and Senses pear, then to stage C (stage 2), where burst of less basement, dark cave, etc.). External zeitge-
can be detected. Drowsiness further descends
only when a person is completely isolated
to sleep stage B (stage 1), where θ waves ap-
from the outside influences (e.g., in a window-
fast waves (sleep spindles) and isolated waves
bers (entraining signals) synchronize the bio-
(K complexes) can be recorded, and ultimately
logical clock to precise 24-hour cycles. It takes
to the stages of deep sleep (stages D/E = stages
several days to “reset” the biological clock, e.g.,
after a long journey from east to west (jet lag).
3/4), characterized by the appearance of δ
waves. Their amplitude increases while their
Important genetic “cogwheels” of the central bio-
frequency drops to a minimum in phase E
logical clock of mammals were recently discovered
(! p. 333 D). This phase is therefore referred to
(! A1). Neurons of the SCN contain specific proteins
old is highest about 1 hour after a person falls
to form heterodimers. The resulting CLOCK/BMAL1
asleep. Sleep then becomes less deep and the
complexes enter the cell nuclei, where their pro-
moter sequences (E-box) bind to period (per) oscilla-
first episode of rapid eye movement (REM) oc-
12 as slow-wave sleep (SWS). The arousal thresh- (CLOCK and BMAL1), the PAS domains of which bind
tor genes per1, per2, and per3, thereby activating
curs. This completes the first sleep cycle.
During REM sleep, most of the skeletal muscles their transcription. After a latency period, expression
of the genes yields the proteins PER1, PER2, and
become atonic (inhibition of motoneurons) PER3, which jointly function as a trimer to block the
while the breathing and heart rates increase. effect of CLOCK/BMAL1, thereby completing the
The face and fingers suddenly start to twitch, negative feedback loop. The mechanism by which
and penile erection and rapid eye movements this cycle activates subsequent neuronal actions
occur. All other stages of sleep are collectively (membrane potentials) is still unclear.
referred to as non-REM sleep (NREM). Sleepers The main external zeitgeber for 24-hour syn-
aroused from REM sleep are more often able to chronization of the sleep–wake cycle is bright
describe their dreams than when aroused light (photic entrainment). Light stimuli are
from NREM sleep. The sleep cycle normally directly sensed by a small, melanopsin-con-
lasts about 90 min and is repeated 4–5 times taining fraction of retinal ganglion cells and
each night (! p. 333 D). Towards morning, conducted to the SCN via the retinohy-
NREM sleep becomes shorter and more even, pothalamic tract (! A2,3). The coupled cells of
while the REM episodes increase from ca. the SCN (! A3) bring about circadian rhythms
10 min to over 30 min.
of hormone secretion, core temperature, and
Infants sleep longest (about 16 hours/day, 50% sleep–wake cycles (! A5, B) by various effec-
REM), 10-year-olds sleep an average 10 hours (20% tor systems of the CNS (! A4).
REM), young adults sleep 7–8 hours a day, and adults
over 50 sleep an average 6 hours or so (both 20% The zeitgeber slows or accelerates the rhythm, de-
REM). The proportion of SWS clearly decreases in pending on which phase it is in. Signals from the
favor of stage C (stage 2) sleep. zeitgeber also reaches the epiphysis (pineal body,
When a person is deprived of REM sleep pineal gland) where it inhibits the secretion of mela-
(awakened during this phase), the duration of the tonin which is high at night. Since it exerts its effects
next REM phase increases to compensate for the mainly on the SCN, administration of melatonin
deficit. The first two to three sleep cycles (core before retiring at night can greatly reduce the time
sleep) are essential. Total sleep deprivation leads to required to “reset” the biological clock. The main
death, but the reason is still unclear because too little reason is that it temporarily “deactivates” the SCN
is known about the physiological role of sleep. (via MT 2 receptors), thereby excluding most noc-
334 turnal neuronal input (except light stimuli).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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