If you're in the Northern Hemisphere - long dark period now - you are possibly showing signs of a condition reported as SAD - seasonal affective disorder.
The photoperiod is not your friend. One solution is at least two hours of very bright light in the evening. Anyway, bright light has far less chance to harm you, other choices to deal with SAD might.
http://www.mayoclinic.com/health/seasonal-affective-disorder/MH00023