Sunshine! A glorious thing, even when temperatures are low. Coming from the subtropics, I think I am particularly susceptible to the presence or absence of sun. I'm 'up' when the sun shines, and 'down' when it's gloomy - I yo-yo back and forth, as the weather changes, the sun's puppet. Light drives my mood.
Right now, in the temperate northern hemisphere, I'm counting the days till the solstice, when daily darkness starts to recede by mere minutes each day. Just 11 days to go till we get a little more daily light, when we hit the planetary limit of how far we can tilt from the sun. It's the turning point I've come to look forward to; it's onward and upward from there (... till next year). I think my personal healthy light limit is more like the time of the equinox - it can start getting lighter for me from there!
I'm lucky enough to have a desk in a sunlit area of my home. It makes it a nice place to be. A nice place to hang out (vegetate?) or work, or read. Light shows up the dirt and dust, though, and light streaming in through my windows makes my computer screen hard to read, but I can live with that!
When there's no sun, my workspace changes character dramatically - too cold to sit at for long, too dreary to feel inspired, too grey to be a source of energy.