Causes
The cause of SAD is unknown. It is believed to be caused by a decrease
in the person's exposure to sunlight. It may be linked to the body's internal
clock, which controls temperature and hormone production.
SAD may also be related to the levels of melatonin in the body, a
hormone secreted by the pineal gland. The nerve centers in the brain that
control daily rhythms and moods are stimulated by the amount of light entering
the eyes. During the night, the pineal gland produces melatonin, which makes
people drowsy. In the morning, the bright light of the sun causes the gland
to stop producing melatonin. On dull winter days, not enough light is received
to trigger this waking up process and for some people it may become very difficult
to get up in the morning.
In addition, on dark winter days, less light comes into the eyes during the
daylight hours, and this may cause some people to feel "low."
Geography seems to play a role. SAD is more common among people who live in
northern climates, or among people who move from a sunny, southern climate to
a more northern climate. Interestingly, people who live in northern climates
where there is little or no snow are affected to a greater degree. It is believed
that this is because snow reflects more light.
SAD can affect anyone, even if they are not already predisposed to depression.
If someone has a history of depression and lives in a more northern climate,
they may be more susceptible.