I think the biggest one for me was sleep habits. I used to be a confirmed night owl with a wacky sleep schedule. Eventually I realized that this was messing up my moods, so I switched to be a morning person with a pretty regular sleep schedule.

It took a lot of small changes, but a key for me was careful regulation of light. I started with a light-based alarm clock, which was useful. These days I have automated lights that dim in the evening and come up in the morning. It turns out I'm just not a responsible lightswitch user; if I leave manual lights on I'll stay up way later and then feel bad the next day. In retrospect, I felt bad a lot, but it was my baseline so I just rolled with it.

These days I'm more even-tempered, more energetic, more productive. I'll still enjoy an occasional very late night, but I'm always careful to return to my usual sleep schedule as soon as possible.