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The causes are still partly a mystery, but
it is known that the nerve centres in our brains controlling our daily
rhythms and moods are stimulated by the amount of light entering the
eyes. During the night, the pineal gland produces a substance called
melatonin which makes us drowsy. At daybreak, the bright light causes
the gland to stop producing this melatonin. But on dull winter days,
especially indoors, not enough light is received to trigger this waking
up process.
More recently, good evidence has been found
to link exposure to bright light with the increased production of a
substance called serotonin, or 5HT. Serotonin is a neurotransmitter,
ie it carries signals from the output of one nerve to the input of the
next inside the brain. Lack of serotonin is known to be a cause of depression,
and indeed this maps well onto what people say: tasks which are easy
when you are well become frustratingly difficult when you are depressed.
Serotonin is the substance made more plentiful by the now well known
drug Prozac, and the similar "SSRI" drugs.It
is tempting to note that melatonin is made from serotonin within the
brain, but there is no evidence to say that this link plays a part in
what happens.
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