Light exposure in the natural environment: Relevance to mood and sleep disorders
Introduction
In addition to being necessary for vision, light also plays a primary role in circadian physiology. Humans are diurnal animals and their biological clock synchronizes their physiological functions such that those associated with activity happen in the daytime while those associated with rest occur at night. Light is the main environmental cue used by the circadian clock to achieve this synchronization with the day–night cycle.
The earliest studies on the non-visual effects of light in humans concluded that bright light, of an intensity found only outdoors during the day, was necessary for circadian entrainment in humans. Subsequent developments involving light therapy for mood and sleep disorders, therefore, focused entirely on exposure to bright light at specific times of day. It is now known that the circadian system is not only able to respond to light signals of low intensity, but that it can also integrate variations in light intensity over the 24-h cycle. A further development is the use of ambulatory monitors, which have enabled researchers to measure continuously the exposure to light of subjects performing their usual activities. Although the first ambulatory studies also focused uniquely on exposure to bright light, contemporary studies using this technology aim at understanding how the entire profile of light–dark exposure can influence the circadian clock and, consequently, mood, sleep, and vigilance quality.
This paper will first present a brief summary of the main facts and concepts underlying the non-visual effects of light. We will then present the results of some of our studies using ambulatory measurements of light exposure in the natural environment, to illustrate how these concepts may apply to real-life situations. Finally, we will discuss the clinical relevance of those research findings for mood, sleep, and circadian disorders.
Section snippets
Non-visual photoreception
The primary pacemaker of the mammalian circadian system is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The SCN contain cells that alter their level of activity according to light intensity in the environment [1]. These cells are sensitive to the global luminance in the environment and can integrate light exposure over relatively long periods of time. The light signal reaches the SCN mainly through a dedicated pathway, the retinohypothalamic tract (RHT). It has been known
Laboratory vs. natural light exposure
In laboratory studies, the circadian effects of light exposure are evaluated in conditions totally different from those experienced in the real world. For example, in the laboratory, subjects can be maintained in near darkness except for the duration of the experimental light exposure, light intensity can be kept constant for the entire length of the treatment (up to 6.5 h in a row), monochromatic light can be used to measure the effects of different wavelengths, and the timing of light exposure
Clinical relevance and future directions
The earliest studies on the effects of light in humans focused only on light intensity. Consequently, the early hypotheses on the contribution of light exposure in physiopathology of sleep and mood disorders, as well as the development of experimental treatments using light therapy, were based mainly on the known effects of bright light exposure. Recent studies on natural light exposure, such as those reviewed in this paper, suggest that it is not only bright light that affects humans. Studies
Acknowledgements
Research was supported by CIHR and NSERC (Canada). C.B. was supported by fellowships from IRSST and Vision Research Network (Québec). We are grateful to Dominique Petit and Victoria Lees for their comments on the manuscript.
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