Sounds like it could be seasonal affective disorder (SAD)
"People with seasonal affective disorder (SAD) instinctively shift their melatonin levels with the seasons, paralleling the hibernation patterns of mammals.
This does not appear to be true for those who don't react to shorter days and longer nights with deepening depression.
In patients who had SAD, the duration of melatonin secretion became longer in winter and shorter in summer, just as it occurs in other mammals. That could be controlling this panoply of changes that occurs in people when they get depressed in the winter.
Seasonal affective disorder, or "winter depression," is a psychiatric disorder that strikes during winter months, when daylight hours are naturally shortened. The disorder in humans seems to mimic the behavior exhibited by hibernating animals, such as increased sleep and decreased activity.
Melatonin, a hormone produced by the pineal gland in response to the darkness of nighttime, promotes sleep and helps regulate the body clock. Hibernation in mammals is triggered when the brain responds to the body's increase in melatonin production."
Article on Winter Depression