What is seasonal affective disorder (SAD)?
Humors, seasons and depression
Around 2500 BC, ancient Greek and Ayurvedic physicians first described how humors body fluids such as bile or blood were related to a persons wellbeing and mood. Thousands of years later in the second century AD, the concept was firmly entrenched in medicine, and physicians such as Galen wrote detailed descriptions on how these humors related to the seasons. In the past 20 years, modern science has confirmed that seasons do affect mood. Scientists now believe there could be a relationship between the humor, yellow bile, and depression.
Seasonal affective disorder (SAD)
In 1984, Dr Norman Rosenthal and his team at the National Institute of Mental Health, USA, described a condition they termed seasonal affective disorder, or SAD. Dr Rosenthal observed there was a link between the decreased hours of sunlight during winter and the occurrence of depression. Most people are aware of the winter blues where tiredness, difficulty in getting out of bed and weight gain become common as autumn turns into winter. But for some individuals these blues can become so exaggerated they pose a serious health problem.
Typical symptoms associated with SAD include a decrease in physical activity, sometimes with almost any effort seeming too much. Associated with this diminished activity is an increase in time spent asleep, a condition called hypersomnia. Often there is an increased craving for high-energy carbohydrate foods, such as cakes. As a result, weight gain is often reported in SAD sufferers.
However, its not the physical symptoms of SAD that are the most debilitating. Common mood changes include depression, anxiety and irritability. These, in turn, can lead to poor concentration, breakdowns in relationships, troubles in the workplace, loss of libido and increased susceptibility to infection. In fact, irritability associated with SAD can be so extreme it may be partly responsible for the increased incidence of child abuse during winter months.
Risk factors for SAD
What makes certain people susceptible to SAD? Without doubt, the most important factor is the persons inherent body chemistry; that is, how their brain chemistry responds to decreases in light. Next come the external factors.
The further you live from the equator, the more likely you are to suffer from the condition, because winters have fewer daylight hours. In fact, any condition that reduces exposure to light during winter increases the likelihood of SAD for example, extended periods of cloud cover and working in an office for long hours. Typical fluorescent office lighting cannot substitute for sunlight. It has even been reported that some individuals who work long hours inside office buildings with few windows may experience symptoms all year round.
Who develops SAD? Surprisingly, the vast majority (70 to 80 per cent) are women, with most developing the condition for the first time in their thirties. Cases of childhood SAD can also occur. Based on two major epidemiological studies in Italy and Switzerland, it appears that around 9-10 per cent of the population is affected by SAD to varying degrees. However, for every individual with full-blown SAD, there are many more with milder winter blues, which can nevertheless still impact on a persons quality of life.
Bright light therapy
Dr Norman Rosenthal pioneered the use of special artificial lights for the treatment of SAD. To date this has proven to be the most successful therapy. However, its important to discuss various treatment alternatives with your doctor in case the depression is due to some other medical condition. The amount of light needed varies from one individual to another. Generally, commercial lightboxes put out 10,000 lux (a lux is a measure of light intensity). This amount of light would be roughly equivalent to outdoor light. The time spent in front of the light depends on the intensity of the light source and the distance one sits from the light. For most people, between 30 and 60 minutes is sufficient to get a response.
Light therapy does take time and regular use. The best time is usually first thing in the morning. Studies have shown that regular fluorescent light works just as well as some of the specialised and expensive full-spectrum lights. Commercially built lightboxes are preferred since the user knows the exact intensity of light and that there are no isolated hot spots that could damage eyes. The distance the patient is from the lightbox is very important, with most people choosing to read a book or eat breakfast while using the light.
One problem is that people with SAD frequently have difficulty getting out of bed in the morning. As a result, compliance with bright light therapy can be poor. Possible options are special light visors and portable lightboxes that can be used in the workplace. Another recent option that has been successfully trialled in Seattle is a lightbox that simulates a summer dawn. The light comes on automatically while the person is still asleep and then gradually increases in intensity until the person awakens.
Potential side-effects of light therapy are rare, but when they do occur most often they include jitteriness, a sensation of eyestrain and headache. Some people may get insomnia when they use the light in the evening. There has been debate on whether there might be long-term eye damage, but none has been documented. However, it is important that lights with proper screening of UV wavelengths are used. Long-term users should only glimpse at the light and never stare at it during use and have a regular eye examination.
Research has shown that SAD symptoms improve when individuals take a one-hour daily walk outside. Outdoor light, even when the sky is overcast, provides as much light as a lightbox or more. But for many people, spending an hour outside each day during daylight hours in winter can prove difficult because of lack of time. Furthermore, its difficult to get motivated on days that are cold and wet. Medications containing Selective Serotonin Re-uptake Inhibitors (SSRI) have been shown to be effective in SAD, but some individuals need a combination of light and medication. Also, as with most depression, daily exercise can be helpful. A double benefit can be achieved by using an exercise regime that involves outdoor activities.
Our biological clock
The actual cause of SAD remains elusive. Discovering the link between mood and light has proven an enormous challenge for scientists. Most of their attention has focused on the biological clock, which helps to synchronise us with the light-dark cycle of the physical world, so we are alert during the day and sleepy at night. We only become aware of our clock when we fly from one time zone to another and experience the unpleasant sensation of jet lag. Most forms of life, including plants, seem to possess some means of regulating biological cycles across the day and across seasons. For example, the Morning Glory flower opens at first light and closes at dusk. However, if you took such a plant on a flight from Sydney to New York, it would open and close its flowers as if it was still in Sydney in other words, a plant with jet lag. After a few days, the pattern of day and night at its new location would reset its biological clock.
If light can affect our biochemistry, it seems reasonable to assume the organ by which we perceive light, the eye, would somehow be involved. Nerve fibres originating from the retinas of the eyes appear to be associated with a special area of the brain called the suprachiasmatic nucleus (SCN). Scientists believe the SCN is probably our biological clock. Dr Gila Lindsley, a sleep specialist, has described how the SCN might work. Light entering the eyes stimulates the SCN.
In turn, the stimulated SCN acts on the pineal gland to reduce the amount of hormone called melatonin that it excretes. As a result, melatonin levels in the body increase at night and decrease during the day. What is the exact role of melatonin in the biological clock mechanism? At the moment there are no clear answers, as this area of research is still in its infancy. It is known that melatonin causes a fall in body temperature, which is exactly what happens when we fall asleep. In a broad outline, melatonin and our biological clock may be related as follows: as darkness falls, more melatonin is released into the blood which possibly triggers a fall in body temperature and sleep. As light returns with the new day, melatonin release is once again slowed, body temperature rises, and we begin to wake.
What does melatonin have to do with the development of SAD? Once again, there are no clear answers, but scientists are continuing to investigate the relationship between melatonin, body temperature and the number of daylight hours. For example, a study published in the Journal of Arctic Medical Research (July 1994) reported that elevated daytime blood levels of melatonin were associated with people susceptible to SAD. They also reported that as daytime levels of melatonin decreased with bright-light therapy, craving for carbohydrates also decreased. This is because carbohydrates create energy and so increase body temperature. Thus the need for SAD patients to use carbohydrates to offset the negative effect of melatonin on decreasing body temperature during the waking hours would diminish.
Why does light therapy work for some people? Scientists have speculated that people who develop SAD may have eyes that are less sensitive to light. Others report there may be an impairment in the suprachiasmatic nucleus in SAD patients, making it less responsive to light. While these theories are interesting, a whole new exciting avenue of investigation has recently begun because of an extraordinary finding. A study published in Science Magazine reported that bright light shone onto areas of the body where the skin is thin could shift a persons sleep-wake cycles; in other words, reset their biological clock. The use of bright lights to treat SAD patients has proven enormously successful, but it has always been considered that the patient should have some sort of visual contact with the light for it to be effective. In this study, though, the light source used was a light blanket, which does not emit any light that could reach the eyes. The researchers could only conclude that light was somehow affecting an unknown substance in the blood that was able to influence the biological clock.
The hunt for this mysterious substance began. Plants have several substances photo-reactive pigments which respond to light. Most people would remember from their school days that plants are green because of a compound called chlorophyll. Its this molecule that allows plants to capture energy from the sun. Theres another pigment in plants, called phytochrome, that interacts with light. It can readily change between two vastly different forms a very biologically active form when plants are exposed to sunlight and an inactive form during the night. By responding to light this way, phytochrome allows plants to tell when it is day or night and what season it is.
Bilirubin the humor yellow bile
Are there pigments in humans that might play a similar role? Interestingly, a part of haemoglobin in red blood cells is strikingly similar to the active part of chlorophyll. Humans have a pigment in their blood almost identical to phytochrome. Dr Dan Oren of Yale University School of Medicine has identified this substance as bilirubin. Bilirubin is a yellowish pigment produced when the body breaks down waste haemoglobin. It is responsible for the yellow colour of some newborns, a condition called jaundice. In ancient Greece, physicians such as Hippocrates referred to bilirubin as one of the four humors that affect temperament and mood yellow bile.
So could bilirubin be a key molecule in setting the biological clock and be involved with SAD? Dr Oren and his colleagues became aware that bilirubin levels tended to go up at night and down during the day in normal people. This variation over the day strongly indicated that bilirubin levels were somehow part of our biological clock. The next step was to determine whether bilirubin behaved differently in people with SAD. To their amazement, they found dramatically lower bilirubin levels during the night compared with normal volunteers. In fact, people with SAD had easily half the amount of bilirubin in their blood during the night.
How could bilirubin be part of the SAD story? It seems it will be some time before scientists have the complete answer. However, it is known that bilirubin reacts to light, which means its capable of controlling the biological clock by changing into an active form, just like phytochrome in plants. Also, bilirubin may protect the brain from damage that has been associated with SAD.
Bilirubin is, in fact, one of the most powerful natural antioxidants in our body and it might be through this function that it provides its protection. Future research will investigate whether increasing bilirubin artificially could be used as a treatment for SAD. It may well be that the link between mood and bilirubin was known more than 2000 years ago. So it seems truly remarkable that modern science is only just now investigating this relationship. Perhaps in our frantic rush for answers we discount too quickly the wisdom of the ancients.