It is thought to be related to changes in the amount of daylight, and is usually brought on by changes in the seasons, but can hit anytime the weather changes significantly for long periods of time.
(It; to be related; to changes in the amount of daylight) (It; is brought on; by changes; T:in the seasons; T:usually)
It It
to be related is brought on
to changes in the amount of daylight
by changes T:in the seasons T:usually