We begin life sleeping around the clock - more or less - and our wake/sleep cycling is regulated mainly by our internal state. Our biological clocks start to get entrained to light and dark cycles in early infancy. After a few days of life, most babies start sleeping a bit longer at night than they sleep during the day, although it can take months or even years for the circadian (daily) clock that regulates our sleep to become constant. Even after the internal clock is stabilized, it can be easily disrupted by many factors throughout the life cycle, often resulting in sleep disorders.
The major external factors that disrupt normal sleep rhythms are: seasonal changes, reduced exposure to daylight, working long or variable shifts, and traveling by jet across several time zones. Normal sleep rhythms can also be disrupted by internal factors, such as physical pain, anxiety and menopausal changes. Sleep cycles are closely tied to internal body temperature cycles, which have their own rhythms that can become disrupted for various reasons.
Sleep disorders can be divided into three types: difficulty maintaining sleep (sleep maintenance insomnia), the tendency to wake up very early and to be unable to fall back to sleep (Advanced Sleep Phase Syndrome), and a pattern of having difficulty falling asleep and getting up in the morning (Delayed Sleep Phase Syndrome).
Those who suffer sleep maintenance insomnia describe fitful sleep; in the morning they often feel as if they have spent several hours - even the entire night - lying in bed without sleeping at all. This problem, often associated with aging, may be related to a general problem making sharp demarcations between day and night. Older people may eat whenever they are hungry, rather than at regular mealtimes, and their day may not follow as regular a pattern as it did when they went to work and returned home at consistent times- rituals that reinforce the 24 hour clock. It is possible that the very biological mechanisms that underlie the aging process interfere with the regulation of sleep and wakefulness. Whatever the causes, sleep disorders tend to become more prevalent with age. It is generally estimated that more than half the population over 65 suffers from chronic sleep disturbance, and the use of prescription sleeping medications is very common in the elderly, despite the fact that medication typically does not resolve the problem in the long run - and often makes it worse. Persistent insomnia diminishes the quality of life because it leads to decreased energy levels, reduced fitness and deterioration of health. Light treatment is a simple way to ameliorate this type of sleep disorder. A groundbreaking study at Cornell University Medical College demonstrated "the effectiveness of timed exposure to bright light in the treatment of age-related sleep maintenance insomnia"- findings that have been replicated many times. Light therapy- although it is not a panacea- can improve sleep quality, and improved sleep generally results in improved mood and activity level during the day.
Like sleep maintenance insomnia, Advanced Sleep Phase Disorder (ASPD) is common in the elderly. While it is possible to accommodate to it, by giving in to the desire to go to bed very early and stay up in the middle of the night, studies at the National Institute of Mental Health and other research centers have shown that, for those who wish to change this pattern, light therapy helps to maintain alertness well into the evening and increases the ability to sleep through the night.
Delayed Sleep Phase Disorder (DSPD) is common in adolescents and young adults; it is the tendency to stay up later and later until the sleep wake cycle is almost reversed from what is considered normal. Because young people need at least as much sleep as everyone else - actually teenagers need more - they usually compensate for sleep loss during the week by sleeping late whenever they can - usually on weekends; this can create a vicious cycle that intensifies their difficulty getting up in the morning. Most people need to operate on a daytime schedule, and day/night reversal creates serious problems for students who miss classes or workers who fall asleep on the job. This is especially true at latitudes where seasonal changes are most extreme, because as winter approaches, the light cues that keep most people in sync with the demands of the external world get progressively weaker and it becomes increasingly harder to resist the tendency of the biological clock to reset itself a little later each day. Even with the arrival of long Spring days, those with DSPS may persist in this sleep pattern and continue to miss opportunities to be outdoors when the light is intense. Light therapy has been shown to be beneficial in ameliorating DSPS. At first it is often difficult to get people out of bed to sit in front of a lightbox, since they oversleep and tend to be chronically late; fortunately, it usually takes only a few days to see results.
Light not only regulates the sleep cycle; it can also have profound effects on mood, fertility, rate of healing and autoimmune response. Natural daylight (sunlight), the "pacemaker" of the body, affects the entire gamut of biological processes in humans, as it does in other animals and, of course, in plants. Sunlight is essential to good health, although we need to protect ourselves from its harmful effects by using sunscreen and dark glasses. When sunlight is unavailable or undesirable, simulated natural daylight can substitute for it and produce many of the same benefits without the risks, because UV light is eliminated.
When there are no markers for night and day, as in a sleep laboratory, people of all ages tend to get out of sync with the external world, which operates on a 24-hour cycle, and to gravitate toward a biologically natural 24.5-hour (approximate) cycle. We all rely on external cues involving the cycling of day and night, as well as many ritualized behaviors which help mark the passage of time, to maintain a fairly constant sleep pattern. These external cues are adequate for some people, even at latitudes where there are significant seasonal changes in hours of daylight or long spells of cloudy weather, when their schedules are variable, or when most of their time is spent indoors. But many others do not get enough light under these conditions to keep their biological clock on a 24-hour cycle, so that it slows down, speeds up or becomes erratic for varying periods of time. And for most people, inadequate exposure to light and exposure to light at the "wrong" time causes temporary sleep disruption (jet lag) when they travel rapidly across time zones. It should be no surprise that light therapy can be used to beat jet lag.
It has long been assumed that light therapy is mediated only through the eyes, although there has been some intriguing research to explore the possibility that there might be other ways to send light signals to the brain, possibly with very intense light focused behind the knees, where large blood vessels are close to the surface. For now, lightboxes are the most reliable way to address the problem of biological rhythm disorders. A properly constructed and calibrated lightbox delivers light of an intensity that provides an effective substitute for natural daylight. It is a non-invasive, cost-effective intervention being used throughout the world wherever light deprivation is a problem, and today many health insurance companies in the USA are willing to pay for lightboxes to treat Seasonal Affective Disorder (SAD) and some chronic sleep disorders.