Seasonal depression and light therapy

This essay was written by Michael A. Ferenczi and was first published in the 1997 Mill Hill Essays.

An eye in the middle of the forehead is a recurring mythical theme in many cultures around the world. In Hinduism, the third eye is an everyday presence in the form of the Pottu, the dot worn on the forehead by the Hindu, which nowadays is mainly seen as enhancing beauty, but which also has a mystical meaning, the spiritual sight or insight achieved through yoga. A Tibetan legend explains the third eye of the God Shiva as resulting from a great flame bursting out of his forehead when, whilst he was trying to meditate, his wife surprised him from behind by putting her hands on his eyes and temporarily blackening his world. It is remarkable that anatomical evidence for the third eye can be found. Fossil fish have the remnant of a third eye in the roof of the skull and in the present day goldfish, the third eye, known as the pineal gland, still responds to the light filtered through the skull. Even in sheep, the pineal gland is directly affected by bright light. In man, this pea-sized gland is deeply embedded in the base of the skull, but microscopical examination reveals that it consists of cells with distinct features of the rod-shaped light-sensitive cells found in the retina, suggesting that the pineal gland originally had a role as a light-sensitive organ. The pineal gland receives signals from regions of the brain directly affected by the signals travelling down the optic nerves, which control the the night/day cycle of hormonal activity and the sleep/wake cycle, the body’s so-called circadian rhythms.

The function of the pineal gland is still unclear. It is known to secrete a hormone called melatonin. The amount of melatonin released by the pineal gland can be measured in the blood. Under normal conditions, melatonin levels are low in the daytime, and rise gradually at night, peaking at two or three in the morning, and gradually decrease until it is time to wake up. The pineal gland and melatonin are thus implicated in the sleep/wake cycle and more and more roles for the pineal gland and melatonin are being suggested, often without real evaluation. Melatonin is acquiring near miraculous properties. One area has received close scrutiny. During recovery from jet-lag or adjustment to new shift times, the pattern of secretion of melatonin which is out of step with the sleep/wake cycle adjusts over five to six days to the new pattern. Here melatonin acts as a marker of the biological clock, but the effects of melatonin on the organs it reaches are unknown.

Through our daily cycle of alertness and tiredness, the biological clock affects our moods and our performance. Superimposed on the daily cycle, there are other biological cycles of various periods, in humans and animals, such as oestrus and winter hibernation. These biological cycles occur naturally, but without external cues, some would quickly be out of step with the real world. Adjustment from jet lag would not happen. As far as melatonin is concerned, it appears that daylight is the necessary ingredient which keeps the melatonin cycle in time with the real world. Without daylight, the body adjusts naturally to a twenty-five hour rhythm, and it is the light reaching the eyes which, in humans, is required for this process. So the eyes are not only needed for creating mental images of the world around us, they also transmit more elementary information which is used for regulating hormone levels. We may not respond to light like plants, but perhaps some of our need for sunlight should not be underestimated.

Apart from the daily cycle of light, we are also subject to the annual cycle of the seasons, with the long days of summer followed by the short days of winter. Again we are subjected to varying degrees of daylight exposure. Again, this cycle appears to have consequences for our moods and our well being. Some people, perhaps as many as one in ten of the population suffer from Seasonal Affective Disorder or SAD, a form of depression characterised chiefly by its seasonality, by depressed moods, decreased energy, a tendency to sleep too much and to feel excessively tired, decreased interest in sex, increased appetite, and craving for sweet foods, particularly chocolate biscuits, which results in weight gain. It is reported that four out of five of these patients are women, with the largest patient group being in their late twenties. It may however be that women are more likely to report this form of depression, and to seek help, than men, or that women’s lifestyle make them more prone to it. In my own correspondence with sufferers, the male/female split seems much more even than that described in the literature. With the arrival of spring and early summer, the symptoms disappear. Energy returns and some of the extra pounds melt away.

What is remarkable about this form of depression is that exposure to bright lights alleviates the symptoms. Depending on the light intensity, as little as half an hour each day causes the worst of the symptoms to pass. The mood lifts, and energy returns. Again, it is light through the eyes which matters. Some studies, but not all, show that patients with SAD have a less pronounced cycle of melatonin secretion than people without SAD, and light therapy appears to restore a normal pattern. The mechanism by which bright lights work is still unknown. For some, bright lights mainly work by strengthening the setting of the circadian cycle. For others, the lights affect a mechanism in the brain known as the serotonin pathway, because drugs which are known to increase the amount of the hormone serotonin in the brain appear also to alleviate SAD symptoms. In any case, the incidence of SAD depends on the length of the day, as explored by studies carried out in Norwegian cities at various latitudes. Norway is an ideal country for studying this effect as it stretches from fifty-eight degrees latitude in the south to seventy-one degrees in the far north, well beyond the arctic circle, and a total distance of one thousand four hundred kilometres. The annual variation in daylight hours extends from a minimum of five at the winter solstice in Oslo, at sixty degrees of latitude, to constant winter darkness from mid-November to late January one thousand kilometres further north in Tromso. As one might expect, SAD is more prevalent in Tromso than in Oslo, and so is the prevalence of winter disorders of eating and sleeping.

Measurements of mood also show differences with the seasons and the latitude, and even sexual interest seems to correlate with exposure to natural daylight. So strong seems to be the effect of daylight on many aspects of our behaviour, that daylight exposure may explain, in part, why the temperament and lifestyles of Mediterraneans and Northerners are so different. Of course other factors such as culture, climate, diet or race may also contribute here. Results of a recent study carried out in Birmingham suggest that women of asian origin are as likely to experience winter depression as the rest of the population, but that asian women who have spent a large part of their lives in more tropical countries suffer from a higher incidence of winter depression when living in the United Kingdom than those who were born here. SAD seems to affect all ethnic groups. Exposure to natural daylight helps in alleviating the symptoms of SAD, but our urban lifestyle often does not lend itself to making the most of the little daylight there is, and often most of our waking hours are spent indoors in semi-darkness. A study in sunny San Diego has shown that on average, Californian forty to sixty year-olds spend less than one hour per day in daylight.

The amount of light reaching the eyes in a living room at night is several hundred times less than what is experienced outdoors on a sunny summer’s day. The scientific unit of intensity is called a lux, and intensity depends on both the power of the light source and the distance from it. The light intensity used during light therapy, up to ten thousand lux, is usually about ten times brighter than exposure to normal room lights but still much weaker than the light experienced on a sunny beach. The response to light is felt within the first week of treatment. The colour of the light used in light therapy does not seem to matter much. In the first trials in the early 1980’s, full spectrum light was used. This is light produced by special fluorescent lamps which release light more similar to that of the sun than conventional fluorescent lamps. The disadvantage of these lamps is their high cost and the relatively large amount of ultra violet radiation they emit. Since then, it has been found that for light therapy, conventional fluorescent lamps are as effective, less expensive and produce much less of the damaging ultra-violet radiation which over a long period of time might be harmful to the eyes. Daily light therapy treatment should start in September and last until late spring. Some reports show that the use of light therapy in advance of the development of winter symptoms is effective. The duration of light therapy sessions depends on the intensity of the light reaching the eyes. In the early 1980’s, treatment consisted of two to three hours of exposure to two thousand five hundred lux. More recently it was found that thirty minutes exposure to ten thousand lux was equally effective, and less disruptive for many patients. The time of day when light therapy is administered is not critical for the beneficial effects to be perceived, which argues against the hypothesis that in the treatment of SAD, light therapy functions by resetting the body clock (the circadian rhythm).

Light therapy has now become an accepted form of therapy, and a new ingredient of daily routine for those who find relief from the symptoms. Treatment is safe. There have been a few reports of patients temporarily developing a mild form of manic behaviour as a result of light therapy, and some incidence of headaches, but these have disappeared after readjustment of the therapy schedule. No deleterious effects on the eyes have been reported. Some do not find relief, and in their case, SAD may be superimposed on other psychological problems, or the diagnosis of SAD may not be appropriate. Attention should be given to the quality of the light used in light therapy. The illuminated area should be large so as to avoid uncomfortable glare, and the presence of ultraviolet rays should be minimised by careful choice of the light source and by the use of filters. Some alternative forms of light therapy have been developed. For example, a dawn simulator used at wake-up time which slowly reduces darkness over a forty-five minute period to a low light level of 100 lux is apparently also effective in alleviating SAD symptoms. A light visor producing constant low light illumination has had mixed results, and was not readily accepted by patients. Of course exposure to natural daylight is highly recommended whenever possible and not surprisingly winter holidays in the sun invariably relieve SAD symptoms. Exercise is also found to be beneficial, but the effects of outdoor exercise may in part result from the accompanying exposure to daylight.

Light therapy is used regularly to treat various forms of sleep disorders and is used also to overcome the effects of jet lag and changes in shift-work schedules. Here, the timing of the light therapy is all-important as it is used to accelerate the establishment of the appropriate pattern of melatonin secretion. Light therapy is used in the morning or before the work shift. In these applications, oral ingestion of melatonin before going to sleep also helps to reset the circadian rhythm, and is found to be effective in promoting sleep. Disruption of sleep patterns are often observed in the very old and in patients suffering from dementia, with poor links between night-time and sleeping time. It has been suggested that light therapy is effective in restoring the circadian rhythms and the sleep pattern in such patients. One possible explanation for the effect of light therapy in this case may be a result of the increased opacity of the cornea in the elderly. Above the age of sixty, the cornea transmits to the retina less than two thirds of the light which it transmits in young people. This means that the light cues are not as marked in the elderly as they are in the younger population, resulting in the possible de-phasing of the circadian rhythms. It is reported that some forms of blindness are also accompanied by loss of circadian rhythms.

In spite of these apparently wondrous effects of light therapy, some healthy scepticism is not out of order. Notwithstanding the large number of studies which demonstrate the efficacy of light therapy in treating the symptoms of SAD and the easy acceptance by the public of a condition which appears to explains the dark moods of winter, no properly controlled study has yet been carried out. This is because a placebo equivalent of the bright lights cannot be designed: the patient always knows whether he or she was, or was not, exposed to bright lights. Double crossover experimental design is the best available, where patient groups are exposed to alternate modes of therapy, and the effects, such as variations in blood melatonin levels, are measured as objectively as possible. Moreover, no satisfactory physiological or biochemical mechanism to explain the effectiveness of bright lights has been demonstrated.

Much work still remains to be done to understand how light therapy modifies our moods. Nevertheless the finding that a safe, drug-free treatment is helpful in treating a common form of depression has been a great boon to all those who have benefitted from the effects of bright lights.


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