These are physical, mental and behavioral
changes that follow an approximately 24-hour cycle, responding
primarily to light and darkness in an organism’s environment. The
study of circadian rhythms is called chronobiology.
The master clock that controls circadian rhythms consists of a group
of nerve cells in the brain called the suprachiasmatic nucleus, or
SCN. The SCN contains about 20,000 nerve cells and is located in the
hypothalamus, an area of the brain just above where the optic nerves
from the eyes cross. SCN controls the production of melatonin, a
hormone that makes us sleepy. When there is less light, like at
night, the SCN tells the brain to make more melatonin so we get
drowsy.
Circadian rhythms can influence sleep-wake cycles, hormone release,
body temperature, and other important bodily functions. They have
been linked to various sleep disorders, including insomnia. Abnormal
circadian rhythms have also been associated with obesity, diabetes,
depression, bipolar disorder, and seasonal affective disorder.
Clock genes contain instructions for making clock proteins, whose
levels rise and fall in a regular cyclic pattern. This pattern in
turn regulates the activity of the genes.
Many of the clock genes and proteins are similar across species,
allowing researchers to make important findings about human
circadian processes by studying the clock components of other
organisms. They have identified genes that direct circadian rhythms
in people, fruit flies, mice, fungi, bread mold, plants, and several
other model organisms used for studying genetics.
Jet lag occurs when travelers suffer from disrupted circadian
rhythms. When you pass through different time zones, your body’s
clock will be different from your wristwatch. For example, if you
fly in an airplane from California to New York, you 'lose' 3 hours
of time. So when you wake up at 7:00 a.m., your body still thinks it
is 4:00 a.m., making you feel disoriented. Your body’s clock will
eventually reset itself, but this often takes a few days. My
circadian clock tells me this is a Happy Friday.