Ungh, I can totally feel this. It's happened to me after doing consecutive months of frequent gym sessions and then dropping off, too.
All summer long I've been able to ride 3-5 times per week, often riding close to 200 miles in a week. Now between the early sunsets, wet weather, and overall business, I'm getting on the bike 1-2 times per week. After a few weeks of this I just feel...like it's easier to feel down about things. Harder to wake up in the mornings. Sometimes even harder to get motivated to go out for a ride when I can. Just a mild overall feeling of depression.
Anyway, just curious if anyone else experiences this as winter approaches and their activity level involuntarily starts to decrease?