Humans, like other life forms, have biological clocks, which have been studied systematically only since the 1920s. In 1972 it was established that the body's internal time-keeping mechanism, located in the hypothalamus, maintains a circadian (circa means approximately, dia means daily) rhythm even when it is removed from the body. We have a natural, but adjustable approximately 25 hour cycle that allows us to be flexible in our adaptation to the external world, and our biological clocks may be advanced or delayed for various reasons. However, these readjustments are often neither smooth nor comfortable for us; in fact, they often cause distress.
Many people are affected adversely by the changing of seasons, most dramatically as the days shorten in the fall, and they develop depressive symptoms which continue throughout the winter. The symptoms are - typically - decreased energy and fatigue, difficulty getting up in the morning, a sense of gloom, increased appetite and carbohydrate cravings (often leading to weight gain,) difficulty concentrating, and anxiety or irritability. Individuals may report only some of these symptoms. The defining characteristic of “seasonality” is that the symptoms disappear when the days lengthen in the spring. Throughout the summer, seasonal people generally enjoy feelings of well-being, although heat and humidity can create seasonal problems unrelated to the amount of daylight. Whenever symptoms interfere with normal functioning in this cyclic, seasonal pattern, the diagnosis of Seasonal Affective Disorder may be appropriate. Many people have some, or all, of these symptoms, but not to the degree that interferes seriously with normal functioning, in which case a diagnosis of sub-syndromal SAD (S-SAD) may be indicated. This latter condition is often labelled "winter blues". Some call it "cabin fever" or "Novemberitis". In Norway, it is called morketiden (roughly translates as 'murky times'.) Whatever name it is given, there is a direct relationship between the prevalence of SAD (and S-SAD) and distance from the Equator. SAD and S-SAD (combined) have been estimated to be as high as 25 or 30% in Northern New England (as compared with 8 to 10% in South Florida.) An NIMH/Walter Reed Army Institute survey found a rate of 8% for SAD and 17.1% for S-SAD in a general population sample in Nashua, New Hampshire (N. Rosenthal, M.D., Winter Blues, NY, Guilford, 1993, p.78.)
Seasonal depression results from two different variables that change simultaneously: the length of the day and the intensity of available light. Both variables change gradually, day by day, and while most people notice the changing length of the days, they are often only marginally aware of the changing light intensity. And yet, in the course of a year, at latitudes where there are major seasonal changes, the magnitude of change in the intensity of light is much greater than the variability in the length of the day. In Boston, for example, a day in June may be 16 hours long, almost twice the length of a day in December, while the intensity of the light at dawn can be thousands of times greater! That is why getting outdoors on a winter day - while it may be helpful and worthwhile - is not a sufficient treatment for SAD; there is simply not enough light out there-especially in the morning - to put a disrupted biological clock back on track.
When seasonal people use light therapy, they generally experience noticeable relief from their symptoms within five days. Sometimes the precise amount of time and the exact distance from the lightbox need to be adjusted, requiring several days of experimentation. Most people need to maintain a consistent, daily schedule of use, preferably starting before the symptoms appear in the fall and continuing until there is sufficient daily natural light in the spring, because symptoms tend to recur when light therapy is disrupted for several consecutive days.
Light therapy, in addition to its effectiveness in treating seasonal depression, has been used successfully to treat other problems: sleep disorders (manifested as difficulties staying awake, falling asleep or staying asleep), difficulties adjusting to shift work or unusual schedules, and jet lag, the disorientation and distress that occurs when people travel rapidly across multiple time zones. Research and clinical trials have also demonstrated the therapeutic value of light treatment for attentional difficulties, premenstrual disorders and bulemia nervosa. Many adolescents experience erratic patterns of sleep and wakefulness, along with a host of other biological and psychological changes, and are often highly responsive to light therapy.
A lightbox for therapeutic use is specially designed to meet requirements determined by extensive testing and research conducted in many countries since 1983. It is not recommended to construct a lightbox at home, as it is necessary to have specific calibration of the light emitted from the lightbox to order to obtain therapeutic results, and there is the potential for electrical dangers as well. Lightboxes should be evaluated for output intensity, comfort and ease of use, and safety. They should contain special compact fluorescent bulbs, which emit light closely replicating natural daylight (early morning spring daylight) which should be powered by electronic ballasts, eliminating the flicker of ordinary fluorescents. These bulbs produce negligible amounts of UV and should be shielded by a diffuser to filter out whatever UV may remain. Because all UV is screened out, light therapy does not provide tanning effects and is not a source of Vitamin D.
Lightboxes produce light of varying intensities, measured in lux, to be obtained at specified distances from the box, and this varies for different models. Some lightboxes will provide a therapeutic result only if used for several hours a day, because the light is not sufficiently intense. Other lightboxes may provide the recommended 10,000 lux - but at an uncomfortably close distance (e.g. 10 or 11 inches or even fewer). The optimal 10,000 lux lightbox is designed for personal convenience and efficiency of use, generally requiring only 15 or 20 minutes a day at a comfortable distance of 20" or more.
Size, portability and attractiveness may affect where, when and how regularly a lightbox is used. Some people prefer a detachable cord, which is also a safety feature. The vertical and horizontal stability of a box may not be obvious on first glance, but it can be assessed by checking its relative proportions. A box that enables the user to adjust the lux levels (achieved with separate switches and fuses) adds versatility and may be an essential feature for some people.
It is recommended that the user sit in front of a lightbox while doing an activity, such as eating, reading or watching TV, eyes open, but not staring directly at it. Many clinical trials have noted an advantage to morning use of a lightbox, although there are important exceptions to this. In any case, a lightbox should be used consistently - typically every day - to be maximally effective.
Light therapy has a direct physiological effect on the body. Researchers have speculated that a major role of the hormone melatonin, secreted by the pineal gland, is to help keep the body's circadian rhythms in synchronization with the light-dark cycle at a given latitude. Darkness stimulates the production of melatonin and bright light effectively shuts down its production, affecting body temperature and sleep patterns. Depending on the timing, light can either advance or delay the need to sleep. These time shifts may underlie the antidepressant effect of bright light, by reversing the tendency to hibernate as the days get shorter. Researchers have also been investigating the changes in the action of neurotransmitters (especially serotonin and dopamine) associated with the therapeutic response to light. There is a consensus in the technical literature that light therapy is an effective, non-invasive intervention, that it is the first-line treatment for seasonality, and that it has worked for many thousands of people. Many insurance companies provide reimbursement for lightboxes, as they are cost effective in the long run, especially when compared with other treatment options.