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DrG's Medisense Feature Article
117062-Circadian_Rhythm
Internal Clocks Run Our Lives
by Ann Gerhardt, MD
May 2017
Print Version
Bottom Line at the Top: Many
bodily functions are controlled by an internal clock in the brain that
cycles us according to day and night. Individual cells of many of
our organs also have their own internal clocks that control
metabolism. People who want to be healthy should heed their
circadian rhythms.
We’ve known for a long time that daily urges to perform bodily
functions, like sleeping, waking, eating and pooping, are determined by
circadian rhythms, which are daily cycles of ebb and flow of a bodily
function. Most of these rhythms are controlled by a master clock
located in a deep portion of the brain in the suprachiasmatic nucleus
of the hypothalamus (SCN).
Sunlight prompts the SCN to trigger a sequence of events which raise
cortisol levels, promoting wakefulness. Darkness causes it to
trigger melatonin release, which tells us to sleep. When deprived
of light, the SCN maintains a daily cycle of a little longer than 24
hours. Exposure to sunlight realigns the SCN to daylight and
night.
Problems with SCN signaling directly cause sleep disorders, which
disrupt our health. Jet lag makes us feel bad for days and
night-shift workers have an above-average risk of obesity and
diabetes. Depressed people have a variety of sleep disorder
patterns, don’t cycle cortisol normally and frequently are
overweight.
Other atypical patterns occur naturally: Adolescents who stay up
very late have a day cycle longer than 24 hours. Older people who fall
asleep in early evening have a day cycle shorter than 24 hours.
Circadian rhythm also affects body weight via tiny ‘clocks’
in cells of liver, muscle, fat, pancreas and intestine.
These clocks are semi-independent of the
SCN: They normally cycle according to food intake instead of sun,
but align with the SCN when their human eats only during daylight
hours. This is optimal for health.
Cellular metabolic clocks are important for sugar metabolism.
They gear up during the day to produce enzymes that ‘burn’
blood glucose but taper off at night. Eating at night leads to
higher blood sugar levels in both healthy and diabetic people.
Higher glucose levels in the evening, when most people are sedentary,
shunts calories into fat.
Weight gain is faster when one habitually eats after dark, and
it’s easier to lose weight and keep it off if we eat only during
daylight. A study of overweight people who shifted their
eating from 15 or more hours per day to less than 12 hours only during
daylight found that they lost weight more easily.
As many as five to ten percent of our genes are regulated in
cycles. Body temperature, blood pressure and heart rate peak in
the evening. We tend to eat different foods at different times of
day. We are most alert in late morning. Coordination,
reaction time and exercise capacity surge through the afternoon into
early evening.
Most aspects of heart and blood vessel physiology follow a cyclic
pattern. Blood pressure, heart rate, and blood vessel
distensibility change according to our sleep-wake cycle.
Blood pressure and pulse fall in the dark and during sleep. If
blood pressure doesn’t decline, it means that blood vessel walls
are not functioning properly. They constrict when they should
relax, possibly causing a stroke or heart attack in the wee hours of
the morning. This occurs in people with diabetes, pre-diabetes, high
blood pressure, deconditioning, advanced age and disrupted circadian
rhythm.
Cellular immune, inflammatory and oxidation reactions also cycle, in
part controlled by cellular clocks and in part by the SCN. Even
the bacterial pattern in our intestine changes with loss of normal
light/dark cycling, affecting intestinal integrity and the whole
body’s immune system.
Current knowledge about circadian rhythms tells us to do intellectual
tasks in the morning, exercise in the afternoon, eat only during
daylight and take blood pressure medicine with dinner and allergy
medication at night. With more knowledge of our bodily rhythms,
we may be able to control our health better by timing activities, food
and medicine. ╣