CD - one reason why your mood is so affected when day goes to night, could be that you are running out of energy at this time.
This time of day is also when food addicts are most likely to binge (that and very late at night.)
i have read it is because this is the time of day when we are coming down from work - it has been a long time since lunch (which often is just a stuffed down sandwich), and the transition time from work to home often just means more work!! No wonder people are depressed at this time.
Try to eat a a snack consisting of high fiber carbs and high protein - like cheese with triscuits, or a plain yogurt with real fruit mixed in (not one with lots of refined sugar). Try eating this about an hour to 30 minutes before you normally feel these "blues" coming on and see if it makes a difference.
Also, as someone else mentioned - check out your lights. If there is a significant difference in the types of light you use at home (such as lots of flouresecnets), then these could be the culprit, too. Try switching to more lamps with softer bulbs, and keep lots of plants and blooming flowers in your home. This can sometimes fool the mind into thinking "outdoors" and ban off some of the S.A.D.
I hope some of this helps.