Light exposure in the natural environment: Relevance to mood and sleep disorders

doi:10.1016/j.sleep.2006.11.008

Sleep Medicine

Volume 8, Issue 6, September 2007, Pages 557-565

https://doi.org/10.1016/j.sleep.2006.11.008 Get rights and content

Abstract

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 in such a way that functions associated with activity happen in the daytime while functions associated with rest occur at night. A misalignment between the endogenous circadian clock and the desired sleep schedule is the main cause of circadian sleep disorders; it may be involved in certain mood disorders as well. Since light is the main environmental cue used by the biological clock to set its own timing in relation to the day–night cycle, inappropriate light exposure can be involved in the physiopathology of circadian disorders. Conversely, when handled properly, controlled light exposure can be used to treat some mood and sleep disorders. While the earliest studies in the field focused solely on exposure to bright light, contemporary studies aim at understanding how the entire profile of light–dark exposure can influence the circadian clock and, consequently, mood, sleep, and vigilance quality. Following a brief summary of the main concepts underlying the non-visual effects of light, this paper presents some studies using ambulatory measurements of light exposure to illustrate how these concepts apply in real-life situations and discusses the clinical relevance of light exposure in the natural environment for mood, sleep, and circadian disorders.

Introduction

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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Light exposure in the natural environment: Relevance to mood and sleep disorders

Abstract

Introduction

Section snippets

Non-visual photoreception

Laboratory vs. natural light exposure

Clinical relevance and future directions

Acknowledgements

Brain Res

Neurosci Lett

Neurosci Lett

Brain Res

Neurosci Lett

Gen Comp Endocrinol

Physiol Behav

Biol Psychiatry

Physiol Behav

J Affective Disord

Biol Psychiatry

J Affect Disord

Psychiatry Res

Appl Ergon

Neurosci Lett

Physiol Behav

Prog. Brain Res.

Biol Psychiatry

Neurosci Lett

Neurosci Lett

Neurosci Lett

Phototransduction by retinal ganglion cells that set the circadian clock

Science

Photons, clocks, and consciousness

J Biol Rhythms

Melanopsin, ganglion-cell photoreceptors, and mammalian photoentrainment

J Biol Rhythms

Evidence for a biological dawn and dusk in the human circadian timing system

J Physiol

Stability of melatonin and temperature as circadian phase markers and their relation to sleep times in humans

J Biol Rhythms

Bright light induction of strong (type 0) resetting of the human circadian pacemaker

Science

Demonstration of rapid light-induced advances and delays of the human circadian clock using hormonal phase markers

Am J Physiol

Light effects on human circadian rhythms: a review of recent Andechs experiments

J Biol Rhythms

Entrainment of the human circadian system by light

J Biol Rhythms

High sensitivity of the human circadian melatonin rhythm to resetting by short wavelength light

J Clin Endocrinol Metab

Light suppresses melatonin secretion in humans

Science

Photic regulation of melatonin in humans: ocular and neural signal transduction

J Biol Rhythms

Human melatonin suppression by light is intensity dependent

J Pineal Res

Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression

J Physiol

Suppression of nocturnal plasma melatonin and 6-sulphatoxymelatonin by bright and dim light in man

Horm Metab Res

Interindividual differences in the responses of serum and salivary melatonin to light

Photostasis: implications for pineal, circadian, and visual photosensitivity