Circadian rhythms
These 24-hour cycles keep you on schedule
Body
Rhythm
Are you sleepy sometimes in the afternoon? Do you seem to handle
physical tasks more easily late in the day? If so, you already know
about circadian rhythms.
Your body has more than 100 circadian rhythms. Each unique
24-hour cycle influences an aspect of your body's function,
including body temperature, hormone levels, heart rate, blood
pressure-- even pain threshold. Understanding how these cycles
interplay is fascinating. And, in some cases, you may be able to
plan your day to take advantage of your body's natural rhythms.
How your body keeps time
In your brain is a type of "pacemaker" called the suprachiasmatic
(soo-prah-ki-az-MAT-ik) nuclei. This area of your brain regulates
the firing of nerve cells that seem to set your circadian rhythms.
Scientists can't explain precisely how this area in your brain
"keeps time." They do know your brain relies on outside influences,
"zeitgebers" (ZITE-ga-berz), to keep it on a 24-hour schedule.
The most obvious zeitgeber is daylight. When daylight hits your
eyes, cells in the retinas signal your brain. Other zeitgebers are
sleep, social contact and even regular meal times. They all send
"timekeeping" clues to your brain, helping keep your circadian
rhythms running according to schedule.
Rhythms control your day
Almost no area of your body is unaffected by circadian
rhythms.
Sleep and wake--It may seem you sleep when you're tired
and wake when you're rested. But your sleep patterns follow a
circadian rhythm.
You're most likely to sleep soundly when your temperature is
lowest, in the wee hours of the morning. You're also most likely to
awaken when your temperature starts to rise around 6 to 8 a.m.
As you age, your brain's "pacemaker" loses cells. This changes
your circadian rhythms, especially noticeable in how you sleep. You
may nap more, have disrupted sleep and awaken earlier.
Temperature--Your temperature is lowest when you're inactive.
And activity can make your temperature rise. But despite these
factors, your temperature also follows a definite circadian rhythm.
In the late afternoon, your temperature can be as much as 2
degrees Fahrenheit higher than in the morning. And it will rise and
fall even if you never see daylight.
Hormone production--Almost all hormones are regulated, to
some extent, by circadian rhythms.
Cortisol affects many body functions, including metabolism and
regulation of your immune system. Its levels are highest between 6
and 8 a.m. and gradually decline throughout the day. If you change
your daily sleeping schedule, the peak of cortisol's cycle changes
accordingly.
Growth hormones stimulate growth in children and help maintain
muscle and connective tissue in adults. Sleep triggers hormone
production, regardless of when you go to bed. Production peaks
during the first two hours of sleep. If you're sleep deprived,
production drops.
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 night.
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 appointment.
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.
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.
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.
