Category: Sleep Disorders

We run our modern lives largely by the clock, from the alarms that startle us out of our slumbers and herald each new workday to the watches and clocks that remind us when it's time for meals, after-school pick-up and the like.

In addition to those ubiquitous timekeepers, though, we have internal "clocks" that are part of our biological machinery and which help set our circadian rhythms, regulating everything from our sleep-wake cycles to our appetites and hormone levels. Light coming into our brains via our eyes set those clocks, though no one is sure exactly how this happens.

But a Johns Hopkins biologist — working in collaboration with scientists at the University of Southern California and Cornell University — unlocked part of that mystery recently. Their study found that rod cells — one of three kinds of exquisitely photosensitive cells found in the retina of the eye — are the only ones responsible for "setting" those clocks in low light conditions. What's more, the study found that rods — which take their name from their cylindrical shape — also contribute (along with cones and other retinal cells) to setting internal clocks in bright light conditions. The study appeared in a recent issue of Nature Neuroscience.

These findings are surprising for several reasons, according to study leader Samer Hattar of the Department of Biology at the Krieger School of Arts and Sciences.

"One is that it had previously been thought that circadian rhythms could only be set at relatively bright light intensities, and that didn't turn out to be the case," he explained. "And two, we knew going in that rods 'bleach,' or become ineffective, when exposed to very bright light, so it was thought that rods couldn't be involved in setting our clocks at all in intense light. But they are."

In the study, Hattar's team used a group of mice which were genetically modified to have only rod photoreceptors, meaning their cones and intrinsically photosensitive retinal ganglion cells — both of them light-sensitive cells in the animals' retinas — were not functional. The team then exposed the rodents to varying intensities of light, measuring the animals' responding level of activity by how often they ran on hamster wheels.

The study results are important because they indicate that prolonged exposure to dim or low light at night (such as that in homes and office buildings) can influence mammals' biological clocks and "throw off" their sleep-wake cycles. Hattar suggested that one way people can mitigate this effect is to make sure to get some exposure to bright day light every day. (The exposure to brighter, natural daylight will firmly reset the clocks to a proper asleep-at-night-awake-in-the-day cycle.)

In addition, the study has possible implications for older people being cared for in nursing homes and hospitals, he said.

"Older adults often lose their rod cells to age, which means that their caretakers would be well advised to regularly and deliberately expose them to bright natural daylight in order to make sure that their natural, biological rhythms remain in sync so their sleep-wake cycles remain accurately set," Hattar said. "Otherwise, they could have sleep disturbances, such as intermittent waking or difficulty falling asleep, not to mention the impact on their appetite and other bodily functions."

Hattar's study was funded by the National Institute of General Medical Sciences.

Journal Reference:

1. Cara M Altimus, Ali D Güler, Nazia M Alam, A Cyrus Arman, Glen T Prusky, Alapakkam P Sampath, Samer Hattar. Rod photoreceptors drive circadian photoentrainment across a wide range of light intensities. Nature Neuroscience, 2010; 13 (9): 1107 DOI: 10.1038/nn.2617

NewsPsychology (Sep. 15, 2010) — The millions of middle-aged and older adults who suffer from insomnia have a new drug-free prescription for a more restful night’s sleep. Regular aerobic exercise improves the quality of sleep, mood and vitality, according to a small but significant new study from Northwestern Medicine.

The study is the first to examine the effect of aerobic exercise on middle-aged and older adults with a diagnosis of insomnia. About 50 percent of people in these age groups complain of chronic insomnia symptoms.

The aerobic exercise trial resulted in the most dramatic improvement in patients’ reported quality of sleep, including sleep duration, compared to any other non-pharmacological intervention.

“This is relevant to a huge portion of the population,” said Phyllis Zee, M.D., director of the Sleep Disorders Center at Northwestern Medicine and senior author of a paper to be published in the October issue of Sleep Medicine. The lead author is Kathryn Reid, research assistant professor at Feinberg.

“Insomnia increases with age,” Zee said. “Around middle age, sleep begins to change dramatically. It is essential that we identify behavioral ways to improve sleep. Now we have promising results showing aerobic exercise is a simple strategy to help people sleep better and feel more vigorous.”

The drug-free strategy also is desirable, because it eliminates the potential of a sleeping medication interacting with other drugs a person may be taking, Reid said.

Sleep is an essential part of a healthy lifestyle, like nutrition and exercise, noted Zee, a professor of neurology, neurobiology, and physiology at Northwestern University Feinberg School of Medicine and a physician at Northwestern Memorial Hospital.

“By improving a person’s sleep, you can improve their physical and mental health,” Zee said. “Sleep is a barometer of health, like someone’s temperature. It should be the fifth vital sign. If a person says he or she isn’t sleeping well, we know they are more likely to be in poor health with problems managing their hypertension or diabetes.”

The study included 23 sedentary adults, primarily women, 55 and older who had difficulty falling sleep and/or staying asleep and impaired daytime functioning. Women have the highest prevalence of insomnia. After a conditioning period, the aerobic physical activity group exercised for two 20-minute sessions four times per week or one 30-to-40-minute session four times per week, both for 16 weeks. Participants worked at 75 percent of their maximum heart rate on at least two activities including walking or using a stationary bicycle or treadmill.

Participants in the non-physical activity group participated in recreational or educational activities, such as a cooking class or a museum lecture, which met for about 45 minutes three to five times per week for 16 weeks.

Both groups received education about good sleep hygiene, which includes sleeping in a cool, dark and quiet room, going to bed the same time every night and not staying in bed too long, if you can’t fall asleep.

Exercise improved the participants’ self-reported sleep quality, elevating them from a diagnosis of poor sleeper to good sleeper. They also reported fewer depressive symptoms, more vitality and less daytime sleepiness.

“Better sleep gave them pep, that magical ingredient that makes you want to get up and get out into the world to do things,” Reid said.

The participants’ scores on the Pittsburgh Sleep Quality Index dropped an average of 4.8 points. (A higher score indicates worse sleep.) In a prior study using t’ai chi as a sleep intervention, for example, participants’ average scores dropped 1.8 points.

“Exercise is good for metabolism, weight management and cardiovascular health and now it’s good for sleep,” Zee said.

The research was funded by the National Institute on Aging.

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The above story is reprinted (with editorial adaptations by newsPsychology staff) from materials provided by Northwestern University. The original article was written by Marla Paul.

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of NewsPsychology or its staff.