Sleep and Circadian Rhythms
Circadian rhythms are 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
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
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.