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Slave to the rhythm.

Life has evolved in environments where light intensity cycles at a rate of 24 hours. It is therefore not surprising that rhythms of about 24 hours, i.e. circadian, are integrated in all processes of life, including gene expression, the biochemical processes in a cell, the complex physiology of an organism, its behavior and cognitive processes. A complex system of central and peripheral oscillators is responsible for this circadian modulation of our bodily functions. A central oscillator resides in the hypothalamic suprachiasmatic nucleus (SCN). Even without any timed input, it shows a clear circadian rhythm in its cellular biochemical, membrane electrophysiological and network interaction properties.

The SCN thus contains a true endogenous pacemaker. However, under normal conditions there is neuronal input to the SCN that changes with the 24-hour cycles of environmental light intensity. This input keeps the biological clock synchronized to the environmental 24-hour cycle. The clock in turn enforces the body's physiological, behavioral and cognitive processes to synchronize to the environmental cycle as well.


With increasing age, the clock becomes vulnerable. Cellular biochemical, membrane electrophysiological and network interaction properties change and make it more difficult for the clock to enforce synchronized 24-hour rhythms in physiology, behavior and cognition. This process is quite unfortunate, because fragmentation of 24-hour rhythms, notably in sleep, may add to the adverse effects of aging itself on brain function and cognitive performance.

In order to maintain proper function, the clock may become increasingly dependent on repeated, regular time cues, of which bright light is the most effective. However, quite unfortunately, the elderly tend to expose themselves to bright environments less frequently, and transmission of the light information from the eyes to the clock is compromised by several degenerative processes. Thus, in addition to a vulnerable clock that has increasingly become dependent on 24-hour cycling input, aging is associated with a lack of input.

To understand the mechanism at a conceptual level, the metaphor of a child on a swing may be helpful. By stretching and contracting the legs at specific times during the oscillation, and thus systematically raising and lowering their center of mass, kids can continue swinging, i.e. sustain the oscillation, as long as they desire. Experienced children can get into such oscillation by themselves without any push. A child that doesn't master the specific leg movement yet is more dependent on a regular push.

In the terminology of the swing metaphor, the elderly may resemble young kids who have not mastered the necessary movements well enough to keep the swing going. Measures that enhance regular input to the clock would be predicted to support the function of the circadian timing system and consequently of the physiology and behavior of the elderly. An age-related disorder where the effect of light treatment has been demonstrated most frequently is Alzheimer's disease. More recently, efficacy has also been shown in elderly suffering from major depression. Light exposure also improves function and well-being in elderly insomniacs and in healthy elderly people. In terms of the swing metaphor, a regular push appears advantageous especially for those that for some reason do not optimally master the optimal leg movements anymore to keep the oscillation going.

As to the practical application of light, several studies suggest that properly timed brief exposures at fixed times in the morning or evening may be effective. More recently, the application of whole-day bright light has become of interest. Not only should electric lighting be considered in environments where elderly live, but also the opportunity to exploit outdoor light. The whole-day approach has the additional advantages that it also promotes alertness and facilitates visual information processing required for, e.g., reading and social interaction, and that it impedes dozing off during daytime. These advantages may add up to improved clock function to concertedly promote sound sleep after having experienced a brighter day.

Editor's Note: This article is based on a presentation given at the IES "Light + Seniors Symposium" earlier this year.

Eus J.W. Van Someren is head of the Department of Sleep and Cognition, Netherlands Institute for Neuroscience, an institute of the Royal Netherlands Academy of Arts and Sciences, Amsterdam, The Netherlands. He is also a professor of neurophysiology, Departments of Integrative Neurophysiology and Medical Psychology, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University and Medical Center, Amsterdam, the Netherlands.
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Author:Van Someren, Eus J.W.
Publication:LD+A Magazine
Date:Sep 1, 2012
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