Ermm.....
There are a lot more potential reasons than just the level of sunlight.
If you feel depressed in winter, you tend to look forward to summer, thinking it will bring you better times. If it doesnt then you are in a position where you are thinking "This is as good as its going to get".
Conversely as you enter into winter most people automatically resign themselves to dark months ahead and a light at the end of the tunnel.
The imagery in the mind is striking and could subconciously assist the depressed through winter but that extra "help" would not be there for you in summer.