Saw a post once saying natural light boosts serotonin or something. When my room's dim, I feel way more tired even if I slept well. A blogger tracked her focus and found she did better tasks before noon with sunlight. Even on cloudy days, opening blinds makes a difference for me. It's like your brain needs that cue to kick into gear. Not saying it works for everyone, but the science is pretty clear.
Yeah the thing about natural light changing your whole work mood, I don't really get that. My own focus comes and goes no matter how bright the room is. For me it's more about what I'm actually working on, or if I took a proper break, not the weather outside my window.