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The exact cause of seasonal affective disorder (SAD) is unknown. The symptoms of depression are thought to be linked to the shorter days of the year during the winter months when the body has less exposure to sunlight.

Experts across the world have been researching SAD since the beginning of the 1980s and have developed several theories about what may trigger the condition. These are discussed below.

Hypothalamus

The amount of sunlight you receive affects some of your body's hormones and brain chemicals. However, the effect that sunlight has on the brain is not fully understood.

One theory is that light stimulates a part of the brain called the hypothalamus, which controls your mood, sleep and appetite. These can affect how you feel.

In people with SAD, lack of sunlight and a problem with certain chemicals in the brain prevents the hypothalamus working properly. The lack of light is thought to:

affect the production of the hormone melatonin

affect the production of the hormone serotonin

disturb your circadian rhythms

Melatonin

Melatonin is a hormone that affects sleep. It is produced by a small gland in your brain called the pineal gland. When it is dark, the pineal gland produces melatonin to make you feel sleepy.

When it is light, the retina (the light-sensitive layer of cells at the back of your eye) converts the light into an electric impulse that travels to the hypothalamus. The hypothalamus sends a message to several of the body's glands, including the pineal gland which produces less melatonin.

During the winter months, some people produce amounts of melatonin that are higher than normal, resulting in SAD symptoms such as sleepiness and low energy levels.

Serotonin

Serotonin is another hormone that affects mood, appetite and sleep. It is also a neurotransmitter, which means that it transmits messages between nerve cells. Fewer hours of sunlight means that less serotonin is produced.

If you have SAD, your serotonin levels may be lower than average during the winter. This may result in the messages between nerve cells not being transmitted effectively and resulting in the symptoms of SAD, such as feeling down.

Circadian rhythm

Your body uses light sources, such as the sun, to time various important biological functions and processes, including your mood, sleep, appetite, digestion and energy levels.

This regulation of the body's functions is known as a circadian rhythm and it usually occurs over a 24-hour period. However, a lack of sunlight can sometimes disrupt the body's internal clock, leading to the symptoms of SAD such as depression and low energy levels.

Scientists already knew the skin tans because it has special cells called keratinocytes that produce the dark pigment melanin and give it to other cells to shield them from DNA damage by ultraviolet (UV) light. But they did not know how UV light kickstarts melanin production in the first place. Dr Cui and colleagues suspected that the whole process starts when UV light triggers the gene p53 into action. They tested this by exposing mice whose keratinocytes lacked p53 to UV light; their skin did not tan. The researchers also explored the possibility that p53 protects the skin against DNA damage in another way - by triggering sun seeking behaviour. Apparently, when people are exposed to UV light, like that of the sun, p53 triggers a process that releases a hormone, b-endorphin, that docks in opiate receptors in brain cells and is linked to feelings of pleasure. According to Dr Fisher, "The number one risk factor for melanoma is an inability to tan; people who tan easily or have dark pigmentation are far less likely to develop the disease." He said that this research shows that p53 "has a powerful role in protecting us against sun damage in the skin." 

Vitamin D sufficiency is required for optimal health, and solar ultraviolet B (UVB) irradiance is an important source of vitamin D

In fact, 90% of our vitamin D is synthesised through exposure to the sun

Vitamin D deficiency is when your body, more specifically your blood, does not have enough vitamin D. Vitamin D deficiency can lead to the following conditions and illnesses:

Rickets - a disease of children and infants that interferes with normal bone formation - a failure to mineralize bone. Bones become distorted and bend wrongly because they are soft. During the first three decades of the 20th century rickets was mainly caused by lack of direct exposure to sunlight. The ultraviolet rays necessary for vitamin D synthesis do not pass through ordinary window glass.

Osteomalacia - softening of the bone caused by demineralization (loss of mineral), mainly loss of calcium from bone. This disorder only affects adults and can cause severe musculoskeletal pain.

Osteoporosis - reduced bone mineral density and increased bone fragility.

Vitamin D deficiency has also been linked to the following diseases:

Hypertension (high blood pressure)

Heart failure, according to US researchers.

Ischemic heart disease

TB (tuberculosis)

Cancer

Periodontal disease

MS (multiple sclerosis)

Chronic pain

SAD (seasonal affective disorder)

Peripheral artery disease

Cognitive impairment

Cardiovascular disease among patients with diabetes

Parkinson's disease (possibly: we are not sure whether Parkinson's causes low vitamin D levels, or whether low vitamin D causes Parkinson's)

Alzheimer's disease, suspected by researchers from the Sunlight, Nutrition, and Health Research Center.

Immune system problems - a University of Missouri nutritional sciences researcher found that vitamin D deficiency is associated with inflammation, a negative response of the immune system, in healthy women.

It was estimated that about 50,000-63,000 annual cancer deaths in the U.S. (10% of all cancer deaths) could be prevented if all Americans had sufficient vitamin D. These findings are based on data in the Atlas of Cancer Mortality Rates for the United States, 1950-94, (cancer.gov/atlasplus/type.html), but are also supported by a number of recent reports that vitamin D plays a very important role in increasing survival once cancer is discovered. These deaths greatly outnumber the annual number of deaths from melanoma (8000) and skin cancer (2000).  In the UK, the preventable cancer deaths with sufficient vitamin D may be as high as 20% since oral intake is low and vitamin D produced from solar UVB is much lower than in the U.S.  In addition, UVB irradiance and vitamin D also provide important health benefits in preventing or ameliorating such conditions or diseases as bone diseases and muscle pain, multiple sclerosis, type 1 and type 2 diabetes mellitus, high blood pressure, etc.