"Early to bed, early to rise, makes a man healthy, wealthy, and wise," according to the rhyme. For many older Americans, "early to rise" may be a familiar but troubling part of their daily schedule. A new study indicates that older healthy individuals do tend to arise earlier in the morning, but that both older and younger healthy people share the same circadian period of a little more than 24 hours, countering the belief that the circadian clock has a 25-hour period that shortens in duration with age.
"The circadian period averages 24 hours and 11 minutes in both young and older individuals," said the study's lead author Charles A. Czeisler, Ph.D., M.D. of Harvard Medical School and Brigham and Women's Hospital in Boston. "However, older people tend to arise earlier in the morning, because the hours at which they can get a good night's sleep, relative to their circadian clock, are much more restricted." The National Institute on Aging (NIA) and the National Institute of General Medical Sciences (NIGMS), two components of the National Institutes of Health (NIH), funded the study, which appears in the June 25 issue of Science . 1
A group of 24 men and women participated in the study and included 11 younger and 13 older study subjects with a mean age of 23.7 and 67.4 years, respectively. In order to measure the length of the circadian period, investigators used a number of carefully designed techniques to minimize the factors which can inadvertently reset the circadian clock, including the use of light.
"Previous studies showed the duration of the human circadian period to be 25 hours. Our study indicates that it is substantially closer to 24 hours with minimal interindividual variability, consistent with similar data from many species," says Dr. Czeisler.
"This study indicates that changes in the circadian clock are not inevitable with age," says Andrew A. Monjan, Ph.D., M.P.H., Chief of the Neurobiology of Aging Branch of the NIA's Neuroscience and Neuropsychology of Aging Program.. "A number of factors may all serve to shift the circadian phase. These include activity, exposure to indoor room light, travel, illness and genetics. In older people, the circadian pacemaker may not be any different than that of their younger counterparts, but something, perhaps light exposure, is interrupting sleep and throwing off the sleep-wake cycle. Their sleep phase may peak earlier."
In humans and many animals, the circadian pacemaker or clock (from the Latin for " around a day "), operates on about a 24-hour basis and regulates sleep. This pacemaker is located deep within the brain's hypothalamus in the suprachiasmatic nucleus where it helps the body keep time. It controls a number of body functions and interacts with the mechanisms controlling sleep. For most people, studies show that the pressure to sleep builds up throughout the day and peaks around 9:00 or 10:00 p.m. At this time, the body's temperature starts to drop and lowers about 1° during sleep. As it starts to rise, around 4:00 am, the likelihood of waking increases. In addition, the pineal gland, located deep within the brain, produces and secretes the chemical melatonin at high levels during the night. A number of factors can affect melatonin secretion, especially many common medications and light. For example, a lamp turned off at bedtime can affect how the pineal gland is stimulated, and the sun's appearance each day begins a chemical process that enables a person to shift from sleep into wakefulness.
Charles A. Czeisler, Jeanne F. Duffy, Theresa L. Shanahan, Emery N. Brown, Jude F. Mitchell, David W. Rimmer, Joseph M. Ronda, Edward J. Silva, James S. Allan, Jonathan S. Emens, Derk-Jan Dijk, Richard E. Kronauer. "Stability, Precision, and Near 24-Hour Period of the Human Circadian Pacemaker," Science, June 25, 1999, Volume 284, pp. 2177-2181.
The National Institute on Aging, part of the National Institutes of Health (NIH), located in Bethesda, Maryland, leads the Federal effort supporting basic, clinical, epidemiological and social research on aging and the special needs of older people. The National Institute of General Medical Sciences, another agency within the NIH, supports basic biomedical research.