Symptoms of depression coinciding with shorter daylight hours can definitely be a problem for many people living in the Northern Hemisphere. And, the further away you live from the equator, the less sunlight exposure time is available for your body to produce ample amounts of vitamin D through your skin. In extreme northern areas the amount of sun basking time is cut rather short, to only about 3 to 4 months out of a year.
For those of you who do live further north, and are feeling lethargic, you are most likely experiencing your body's inability to properly regulate the hormone melatonin. The hormone melatonin is produced by the pineal gland. Basically, this hormone is produced during darkness to help induce drowsiness, enabling you to sleep.
On the flip side, exposing yourself to more unfiltered sunlight, during daylight hours, helps with the regulation of an alertness hormone, called serotonin. When there is an adequate amount of this naturally occurring body neurotransmitter chemical being produced, it promotes a sense of alertness and optimal emotional well being.
A useful tip worth mentioning here, this light needs to come in through your eyes as well as your skin. When you are outside on a sunny day, take out the contacts, lay your eyeglasses aside, and for heavens sake, take off those stylish sunglasses for just a little while. Even if you can't see very well without your prescription eye ware, give yourself at least 30 minutes, or longer if you can, to help your body regulate its serotonin production levels.