It is a chemical thing! You are getting less sunlight, working under bad lighting and for the most part, less active. I have talked about neurotransmitters and their involvement in mood in the past. You can look at previous blogs and my article in the Art of Well being : http://www.artofwellbeing.com/articles/biggest_casualty_of_stress.php
The other considerations are Vitamin D3. It is called the Sunshine vitamin. You get it from skin, retina and pineal exposure to sunlight. It is very difficult with a busy lifestyle to get this during the late fall and winter. I test D3 levels and it is rarely in the acceptable range. You need to get 4,000-10,000 i.u. daily during these darker seasons. D3 is much different than D2! - You need D3! It will also keep your immune system happier during the flu season. I find this to be a helpful part of keeping mood up during the winter as well!
There are also full-spectrum lighting that you can put in your office and home to trick the brain into thinking you are getting sunlight. My mother used these at our old office and I could definitely feel a difference when I flipped up the desk lamp. You can even find it in a dawn simulator option, helping keep balance with the diurnal/nocturnal melatonin cycle. (wake/sleep cycle).
Of course, I can't complete this post without mentioning the benefits of exercise! You don't always want to but don't you feel invigorated after a good exercise?! I do!! Ever hear of endorphines? They are the chemicals that give you that "natural high" when you exercise. Exercise also releases adrenaline, serotonin, and dopamine. These chemicals work together to make you feel good. Exercise also helps to lower the stress hormone, cortisol and helps with prevention of heart disease, and a myriad of other diseases.
Need guidence to bring balance for this? That's what I am here for! All you have to do is ask for help!