Clinical Geriatrics

Proper lighting is necessary for vision and to carry out one’s daily activities. This aspect of light-light for vision - is well known by the general public and health professionals. What is less well appreciated is the large and growing body of scientific literature on light as a therapeutic agent. Indeed, although much has been documented over the past decades about its therapeutic uses, light is rarely prescribed by physicians and other healthcare providers. In part, this is because much is yet to be understood about the optimum characteristics of light affecting our health and well-being. Published research studies have employed a wide range of light sources, quantities, spectra, and delivery schedules, without yielding a consensus as to how to optimally provide this treatment. However, underuse of light as therapy is also due to the fact that light is naturally available, and thus unpatentable, making it unappealing as a research or marketing venue for industries.1

Sleep disorders and depression are extremely common in older persons, and in research trials, light therapy has shown some benefits for treating these disorders. Although drug treatments are available for both conditions, in each case the available drugs are not always effective and carry with them the risk of adverse effects. Therefore, healthcare providers treating older persons might benefit from considering light therapy as an alternative or adjunct therapy for selected patients with sleep disorders or depression. In this article, we review the physiology, scientific evidence, therapeutic uses, and cautions regarding light as a therapeutic agent for these common geriatric conditions, and provide recommendations for clinicians.

Physiology of Light as Therapy
The physiological foundation of light as therapy for sleep-wake cycle disturbances is based on the understanding that living organisms have adapted to the daily rotation of the earth by developing biological rhythms that repeat at approximately 24-hour intervals (ie, circadian rhythms); the most prominent physiological manifestations of these are the sleep-wake cycle and diurnal variations in melatonin secretion, cortisol, and core body temperature. The regulation of melatonin and temperature is controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain, which fluctuates cyclically but needs an external stimulus to maintain a consistent 24-hour cycle. That key external stimulus is variation in light and darkness over the course of the day and, in particular, exposure to high light levels during the morning hours. These cyclical changes in light levels in turn synchronize the SCN to the 24-hour day, assuring the timing of physiological rhythms, so that we are asleep during nighttime hours and awake during daytime hours.

Impairment of these SCN-mediated circadian rhythms becomes increasingly common with advancing age, diminished health, and Alzheimer’s disease, thereby contributing to the high prevalence of sleep disturbances in these populations.2-4 Older adults have shown depressed melatonin rhythm amplitudes and alterations in the body’s temperature cycle, which may explain the sleep disorders commonly found in these individuals.5 Thus, the mechanism of light therapy for sleep disorders could be via stimulation of the SCN, effectively correcting the timing and/or strengthening the amplitude body’s circadian cycles.