Cardiovascular system--More strokes and heart attacks occur in
the morning than at any other time of day. This makes some people wonder
if morning exercise is safe.
But experts contend morning changes in your body--not exercise-- may
be responsible for cardiovascular problems. Blood clots most rapidly at
about 8 a.m.
Blood pressure also rises in the morning and stays elevated until
late afternoon. Then it drops off and hits its lowest point during the
These changes occur independently of physical activity. Exercise at
any time of the day is beneficial.
On the other hand, if you're training for athletic competition, you
may have reason to schedule that event later in the day. Athletes seem
to perform best in the late afternoon, when strength, body temperature
and flexibility peak.
Pain tolerance--Athletes who compete late in the day may perform
better because they can "gain" without as much "pain." Pain tolerance is
highest in the afternoon. One study shows tooth pain is lowest in the
late afternoon, a consideration when you schedule your next dental
Medication--Scientists are looking at how circadian rhythms
affect the way your body uses medications. One finding is that less
anesthesia is needed to cause analgesia or drowsiness when administered
in the afternoon.
Experiments with cancer medications are trying to find the time of
day when the drugs are the most helpful with the fewest side effects.
Sex --The majority of sexual
encounters took place at bedtime (11 pm to 1 am). The most common
explanations for this temporal pattern is the biological clock has a
time set for sexual arousal.
Heart attacks--The chances of suffering heart problems are not
equal throughout the day. Heart attacks occur more often around 10
o'clock in the morning than any other time, a peak that previously was
attributed to daily behavior patterns getting underway
Stay on schedule
Changes in daily habits such as a short night's sleep can disrupt
your circadian rhythms. You may be able to stay "in sync" by keeping a
consistent daily schedule.
Most totally blind people have circadian rhythms that
are "free-running" (i.e., that are not synchronized to environmental
time cues and that oscillate on a cycle slightly longer than 24 hours).
This condition causes recurrent insomnia and daytime sleepiness when the
rhythms drift out of phase with the normal 24-hour cycle.10-mg dose of
melatonin was given daily one hour before bedtime will achive a normal
ryththm.. The dose was then reduced to 0.5 mg per day over a period of
three months; the entrainment persisted, even at the lowest dose.
Circadian rhythms are internal body clock
cycles which control regular changes in
mental and physical characteristics that occur in the
course of a day (circadian is Latin for "around a
day"). Most circadian rhythms are controlled by the
body’s biological "clock." This clock, called the
suprachiasmatic nucleus or SCN, is actually a
pair of pinhead-sized brain structures that together
contain about 20,000 neurons. The SCN rests in a part of
the brain called the hypothalamus, just above the
point where the optic nerves cross. Light that reaches
photoreceptors in the retina (a tissue at the
back of the eye) creates signals that travel along the
optic nerve to the SCN.
Signals from the SCN travel to several brain regions,
including the pineal gland, which responds to
light-induced signals by switching off production of the
hormone melatonin. The body’s level of melatonin
normally increases after darkness falls, making people
feel drowsy. The SCN also governs functions that are
synchronized with the sleep/wake cycle, including body
temperature, hormone secretion, urine production, and
changes in blood pressure.
By depriving people of light and other external time
cues, scientists have learned that most people’s
biological clocks work on a 25-hour cycle rather than a
24-hour one. But because sunlight or other bright lights
can reset the SCN, our biological cycles normally follow
the 24-hour cycle of the sun, rather than our innate
cycle. Circadian rhythms can be affected to some degree
by almost any kind of external time cue, such as the
beeping of your alarm clock, the clatter of a garbage
truck, or the timing of your meals. Scientists call
external time cues zeitgebers (German for "time