Hormones 2016

Sleep
Sleep fragmentation results from the increase in overnight arousals and
may be exacerbated by the increasing number of medical conditions related
to old age, including sleep apnoea (interrupted breathing during sleep),
musculoskeletal disorders, and cardiopulmonary disease.
Circadian Rhythms That Influence Sleep
Biological variations that occur in the course of 24 hours are called circadian
rhythms. Circadian rhythms are controlled by the body’s biological clock.
Many bodily functions follow the biologic clock, but sleep and wakefulness
comprise the most important circadian rhythm. Circadian sleep rhythm is
one of the several body rhythms modulated by the hypothalamus (a part of
the brain).
Light directly affects the circadian sleep rhythm. Light is called a "zeitgeber",
a German word meaning time-giver, because it sets the biological clock. A
practical purpose has been proposed for the circadian rhythm, using the
analogy of the brain being somewhat like a battery charging during sleep
and discharging during wakefulness.
Body temperature cycles are also under control of the hypothalamus. An
increase in body temperature is seen during the course of the day and a
decrease is observed during the night. The temperature peaks and troughs
are thought to mirror the sleep rhythm. People who are alert late in the
evening (evening types) have body temperature peaks late in the evening,
while those who find themselves most alert early in the morning (morning
types) have body temperature peaks early in the evening.
Melatonin (a chemical produced by the pineal gland in the brain) has been
implicated as a modulator of light entrainment. It is secreted maximally
during the night. Prolactin, testosterone, and growth hormone also
demonstrate circadian rhythms, with maximal secretion during the night.
Circadian rhythms can be affected to a certain degree by almost any kind of
external stimulus, for example, the beeping of the alarm clock or the timing
of meals. When we cross time zones, our circadian rhythms get disrupted
leading to jet lag. It usually takes several days for our body rhythms to
adjust to the new time.
Symptoms similar to those seen in people with jet lag are common in
people who work during nights or work in shifts. Because these people’s
wake time conflicts with powerful sleep-regulating cues like sunlight, they
often become uncontrollably drowsy during work or may have difficulty
falling asleep during their off time. Their biological clock wants to do one
thing while they are doing something entirely different. People working
in shifts have an increased risk of heart, gastrointestinal, emotional, and
mental problems. All these problems may be related to the disruption of
the circadian sleep rhythm.
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