One word: sunlight.
I end my work day around 4PM. The sun is already down, and it's dark.
Being an early riser don't really help me get sunlight.
That's my work schedule. I'm naturally a night owl. I prefer to work all night, but I can't.
However, I'm also one of the few who doesn't care that we don't see the sun for month during winter. So being a night owl really helps me from getting "SAD".
FYI, during winter: Sunrise: 7:34 AM Sunset: 4:22 PM
Working all early morning and all late noon? Meh.