Once in a while, everyone has trouble getting to sleep. (I rarely do, but that’s because, now that I’m married, I have to get up much earlier than I did in my single days, in order to get my wife off to work. Marriage as a cure for sleeplessness might seem a little drastic to some.) Older people, in particular, sometimes find it difficult to get a good night’s sleep.
A new study casts new light on that problem.
We each have an internal clock, which our bodies need to regulate many functions, of which sleep is the most obvious. In most people, the impulse to sleep peaks around 10 p.m., when the body temperature stops dropping. Body temperature rises again about 4 a.m., making it more likely we’ll wake up, but two hormones, melatonin and cortisol, help keep us asleep.
Earlier studies had indicated that our internal clock ran on a 25-hour cycle. This latest study, however, reveals that the clock actually runs at right around 24 hours (24 hours, 11 minutes, was the average). More importantly, it reveals how that clock can be messed up, making us wakeful when we should be sleepy, and vice versa.
In the study, conducted by Dr. Charles A. Czeisler and colleagues from Harvard Medical School and Brigham and Women’s Hospital in Boston, 24 men and women lived for a month in subdued lighting, with no clues to the passage of time. They were placed on a 28-hour wake-sleep cycle to disconnect their internal clocks from the kind of influences that normally control our days, such as work schedules. Their body chemistry and temperature were monitored. Eleven of the subjects were young men, with an average age of 23.7 years, and 13 (nine men and four women) were much older, with an average age of 67.4 years.
In addition to revealing that the internal clock is much more closely aligned with the actual length of a day than had been thought, the study showed that the internal clock for older people runs at almost exactly the same pace as that of younger people. This startled the researchers, who believed going into the study that the internal clock ran at different times in different people, particularly older people. They hoped to prove this hypothesis and move closer to an explanation of why older people sometimes have difficulty sleeping. Instead, they disproved it (an excellent example of the scientific method at work!).
What they discovered instead was that the most important influence on the human biological clock is light–just as it is for many other animals scientists have studied, from the sea slug to the mosquito. In fact, humans are so sensitive to light, that the earlier studies that indicated a 25-hour internal clock were probably thrown off by light exposure.
Nor does that light have to be shining in our eyes. Other recent research has indicated we have previously unsuspected light detectors throughout our bodies, so that even shining a light on the back of someone’s knees can effect their internal clock. Staying up at night with artificial light pushes the clock forward, putting off when you feel sleepy, but also pushing off the release of melatonin and cortisol, which means it will be much later the next morning–probably after you’ve had to get up–that you finally feel awake.
Elderly people may have trouble sleeping at night because their internal clocks have become unusually sensitive to light. In some people, even lights as dim as the glow from a television can disrupt the sleep/wake cycle.
Better understanding of our biological clocks could eventually benefit people who suffer from more than just insomnia. Your entire body undergoes significant internal changes on a daily basis. Blood pressure, for example, rises in the morning, remains elevated during the day, then decreases during sleep. People with chronic diseases often find their symptoms, too, repeat on a regular cycle.
This means our internal clocks can influence the results of medical tests and the effects of drugs. Increasingly, doctors try to monitor symptoms over a 24-hour period–and may suggest that if the symptoms are worst at a particular time of day, that’s when you should be taking your pill.
The more we understand about our internal clocks, the better we may be able to treat a whole host of diseases and conditions, as well as ensure that everyone gets a good night’s sleep–even single people.
