One word: sunlight.

To expand: Sunlight makes the body stop producing melatonin and start producing serotonin, which improves your mood and alertness. Being deprived of sunlight for extended periods can result in seasonal affective disorder.

Here in Canada, during the winter, I wake up at 6AM. I'm a bit of a early riser I guess. It's dark out.

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

Swedish here. I just hate the way things are set up, so that in the winter you practically don't get any sunlight at all. You go to work in the dark, work inside (not necessarily by a window) and then you go home in the dark. I would much rather shift the day so that the sun is up when I'm not at work. No wonder people get depressed.

Yes! But even with a free schedule, I have no idea how I could split it to work.

Working all early morning and all late noon? Meh.

Which you can still get while asleep, which possibly compares well to people who work all day in only artificial light in big buildings.

For vitamin D, sure, but not for other psychology-related issues. The problem with office workers not getting enough sunlight is very real, though, especially in winter months.