Molecular "clocks" in the brain create natural cycles in many body traits, such as blood pressure and temperature. Scientists have learned that these clocks, which can be reset by sunlight, are controlled by special genes. Knowledge of these clocks is leading to an improved understanding of biological cycles and new ways of treating disorders such as insomnia.
Are you a "night owl" or an "early bird?" The answer depends on a biological "clock" in your brain. This clock controls many natural body cycles -- from the time you wake up each morning to rhythmic changes in body temperature and blood hormone levels.
Nearly all organisms, from bacteria to plants to humans, have biological clocks that help maintain rhythms.
By studying these clocks, scientists are beginning to understand:
- The biological foundations of behavior.
- Jet lag, insomnia, mental disorders, and how to treat them.
- Rhythmic changes in heart rate and other traits that affect the diagnosis and treatment of many disorders, including fever and high blood pressure.
During the mid-1900s, scientists began to examine biological cycles in several different organisms. By the early 1960s, they showed that daily, or circadian rhythms -- "circa" means around and "dia" means day -- are generated internally and synchronized to the 24-hour day. How are these cycles generated and altered?
In mammals, including humans, a biological clock resides in a region of the brain's hypothalamus, a quarter-sized structure that regulates hormone levels and plays a role in emotions. In some insects and snails the clocks are usually located in the retina of the eye. In birds the clocks can also be found in a brain region called the pineal gland or in the hypothalamus.
The clocks are almost always linked to some form of light-sensing cell called a photoreceptor. This type of cell responds to sunlight in ways that help synchronize the clock with the 24-hour day.
Scientists have learned that exposure to light at certain times in the internal cycle can reset the clock in animals. In mammals, light turns on important genes and affects sleep patterns, alertness, and body temperature. In nature, this light sensitivity helps organisms synchronize their clock within the cycle of day and night.
Researchers have found that exposure to strong artificial light at certain times can reset the clock in ways that relieve insomnia, jet lag, and other problems. Light at the wrong time of the internal cycle, however, might contribute to or intensify these conditions.
By studying mold, flies, mice, and other organisms, scientists have learned that the function of the biological clock is controlled by specific genes. Research in flies with disabled or altered clock genes shows that the proteins clock cells produce often work in a negative feedback cycle. When high levels of clock proteins are present, they block further production of these molecules until the levels fall again.
Some clock genes also regulate or stabilize other genes' activity. The way these clock proteins affect body functions is not yet known. However, learning how normal cycles affect the body may improve the diagnosis of many disorders, such as fever and high blood pressure, by accounting for daily rhythmic variation in hormone levels, blood pressure, temperature, and other traits.
Researchers believe only a few genes regulate circadian cycles in most organisms. Mutations in known clock genes usually create large changes in length of the cycle.
Organisms with some mutations are unable to maintain any normal rhythm at all. If mutations in human clock genes act in a similar way, they may prevent some people from synchronizing their cycles with the environment, causing sleep disturbances and other problems.
Scientists have now uncovered several ways to treat clock-related disorders. Some studies suggest that the hormone melatonin, given at specific times, may be useful for resetting daily rhythms to help overcome the effects of jet lag and sleep disorders.
Exposure to bright light at certain times in the cycle may also help people with depression and other disorders. These strategies, now being tested in humans, may brighten the lives of millions of Americans.
 Light causes the brain's internal clock to reset its cycle. Sunlight reaching photo receptors in the retina travels to the brain by the optic nerve. It sets off reactions in a region of the hypothalamus called the suprachiasmatic nucleus (SCN), which serves as the body's clock. Circadian changes in the SCN affect the nervous system and cause daily fluctuations in many body traits. Nerve fibers also carry signals from the SCN to the pineal gland, which affects hormones and other functions. |
Illustration by Lydia Kibiuk, Copyright © 1995 Lydia Kibiuk. |
Kenapa aku post entry nih?? Dah 2 minggu (lebey kurang la) aku takleh tido slagi blum kul 4 pagi!! Bangun pagi mesti dah tengahari (thanx to my room yang kalau siang pun maca malam lagi). I think i need to ask for help la. HELPPPP!!!!
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