Physio Chapter 11 Sleep and Waking
Arepeating cycle of about 24 hours.
Together, the interplay of sleep and waking cycles follow circadian, or daily, rhythms.
To establish and maintain these rhythms, internal biological clocks interact with stimuli known
as zeitgebers: an external cue for setting biological rhythms
Light is the most important zeitgeber for human beings. In the absence of natural light,
human free-running circadian rhythms (Arhythm that is not synchronized to
environmental time cues) last approximately 24.2 hours to 24.9 hours
Exposure to sunlight each day helps reset, or entrain, the internal biological clock to the 24-
hour cycle of the earths rotation.
Totally blind people and sailors on submarines experience free-running cycles that are
longer than 24 hours, often resulting in severe sleep disruptions
In addition to light, other zeitgebers include physical activity, feeding, body temperature,
and sleep-related hormones discussed later in this chapter
Variations in Sleep Patterns
People who are most alert and productive in the morning have been referred to as larks,
whereas night people have been referred to as night owls
Regardless of normal lifetime sleep patterns, nearly everyone acts like an owl during
adolescence because Melatonin, one of the neurochemicals involved in the regulation of sleep
patterns, drops dramatically at the onset of puberty
Following adolescence, many temporary owls will revert to their previous state, possibly due to
the maturation of neural systems that regulate sleep.
Regardless of the origins of adolescent owl behavior, accommodations can be useful. Ex:
Shifting from a 7:15 a.m. start time to an 8:40 a.m. start time improved both attendance and
student grades at Minnesota high schools (Wahlstrom, 2003).
Shift Work, Jet Lag, and Daylight Saving Time
When work demands and circadian rhythms do not match, the consequences can be challenging
and possibly dangerous.
Between 40 and 80 percent of workers on the 11 p.m. to 7:30 a.m. night shift experience
disturbed sleep and a cluster of symptoms referred to as shift maladaptation syndrome
These workers obtain 1.5 hours less total sleep than workers on other shifts, leading to
frequent health, personality, mood, and interpersonal problems.Accidents are also highest
during these over night shifts
Night shift workers are more likely than other workers to develop breast cancer
Shift workers are not only a risk to themselves, but their errors also jeopardize the public.
Hospital workers, such as nurses, are much more likely to make significant errors during
evening or night shifts than during day shifts
Conflicts between internal clocks and external zeitgebers also result in the unsettling experience
of jet lag: After crossing time zones, people often experience fatigue, irritability, and sleepiness
Chronic jet lag might have more serious consequences.Airline flight attendants who crossed
time zones at least once a week for four or more years had reduced reaction times and made 9
percent more mistakes on memory tasks than local crews who did not cross time zones Internal Clocks
The bodys internal master clock is the suprachiasmatic nucleus (SCN) in the hypothalamus
The SCN is only active during the day. This is true regardless of whether a species is diurnal
(awake during the day), like monkeys, or nocturnal (awake at night), like rats.\
The SCN helps animals distinguish between day and night, but other structures dictate
whether an animal is nocturnal or diurnal in its behavior
The SCN is not dependent on input from other structures to maintain its rhythms.
Isolated SCN tissue cultures continued to show rhythmic fluctuations in activity consistent
with the source animals previous daynight cycle
Transplants of SCN tissue also support its role as a master internal clock
It is possible to breed hamsters with short free-running cycles of about 20 hours, in
comparison with the normal hamster cycle of about 24 hours
When SCN tissue from a short-period hamster is transplanted into a normal hamster, the
normal hamster shows the short free-running cycle.
When SCN tissue from a normal hamster is transplanted into a short-period hamster, the
hamster shows normal 24-hour cycles
The SCN acts as a master clock that coordinates the activities of other internal, peripheral
clocks that exist in most body cells. Cells from the SCN, liver, lung, and muscle of rats were
observed following six-hour phase shifts in the ratslight-dark schedules
The SCN adjusted to the new time after only one or two cycles of light and dark, but
peripheral clocks in the other tissues were much slower to respond
Lung and muscle tissue required 6 cycles to adjust to the new time, and the liver required
more than 16 cycles. The effects of phase shifts on muscles, lungs, and other tissues appear
to last long after the initial discomfort is gone
The rhythms of the SCN are heavily influenced by the presence of light. In contrast, the
peripheral clocks are more easily influenced by daily feeding cycles
Many travelers attempt to compensate forjet lag by immediately adjusting their
mealtimes to their current time zone
The Cellular Basis of Circadian Rhythms
three separate genes and their protein products that are involved with cellular circadian rhythms
Tim, Per, and Clock
Together, per and tim proteins inhibit the Clock protein, whereas the Clock protein
promotes the production of more per and tim proteins.
Consequently, as levels of per and tim proteins increase, inhibition of the Clock protein
ensures that no further per and tim proteins will be produced.
As levels of per and tim proteins drop over time, the reduced inhibition of the Clock protein
results in increased production of more per and tim proteins
Neural activity reflects the oscillation of the levels of these internal proteins, providing a
mechanism for communicating rhythms to other cells.
Biochemistry and Circadian Rhythms
The SCN both regulates and responds to the hormone melatonin, an indoleamine secreted by
the pineal gland
Lesions of the SCN abolish the circadian release of melatonin. Melatonin levels are very low
during the day, begin to rise in the hours before sleep, and usually peak at about 4 a.m., a time
when nearly everybody finds it very difficult to stay awake Totally blind individuals experience a melatonin peak at a different time each day, often leading
to sleep difficulties. People with pineal gland tumors or other medical conditions affecting
melatonin report sleep problems
Melatonin release is suppressed by light;Although bright lights are more likely to suppress
melatonin, dimmer lights typical of indoor lighting also have the ability to suppress production
Melatonin supplements have been reported to improve cases of jet lag, shift maladaptation
syndrome, and other sleep disorders
During the winter months at higher latitudes (areas closer to the poles of the Earth), the
reduction in daylight hours can interfere with circadian rhythms. Consequently, some people
will experience a type of depression known as seasonal affective disorder (SAD).
Serotonin levels typically drop in the fall and winter, and people vulnerable to SAD might
experience a greater than normal decrease, leading to symptoms of depression
SAD might also be influenced by disruptions in melatonin release caused by uneven patterns of
SAD is treated by exposure to bright lights, with or without melatonin and antidepressants.
Light therapy administered at dawn corrects cases in which people stay up too late, whereas
light therapy in the evening helps people who are sleepy too early
Icelanders experience lower rates of SAD; might enjoy protective