My biggest health issue once school starts is always how much time I actually spend working during the average week! Lots of research shows that, after 50 hours of work, one's productivity pretty much plateaus. However, I often find myself needing to work more than that, especially if I'm creating a new course. Here are some things I do to keep it fresh!
1. Some sort of exercise right after school gets out. Even if it ends up being walking around the hallways of the school for 10-15 minutes, taking a bit of time to stretch my legs after the school day (not that I sit down much anyway, but still, there's some difference between sustained walking around and the stop-and-start walking I do in the classroom) helps get my energy back up for the afternoon and creates a solid break for my brain as well. If I can help it, I try NOT to think about school things during that time!
2. Go home early when I find I'm not being productive. On those days when it's taken me 2 hours to do 30 minutes worth of work, I just leave. The papers will be graded much faster tomorrow after school once I've had some actual time away from them than they would be that day and the kids can usually wait one more day to get their stuff back.
3. Keep healthy food at school. Especially since I'm often at school late, I'll find myself either snacking incessantly or needing to eat dinner while I'm here. Keeping healthy things in the fridge or my office goes a long way to make sure I'm not getting my nutritional intake from the vending machines.
4. Share something with your students that doesn't have to do with what you teach. Sometimes the extra hours put me into a mental space where I'm basically no longer a human being, and instead my whole identity is that of teacher. The students generally love it when they get to find out that I'm an actual human being, and it helps remind me of that as well! It can also be a good way to get them talking about themselves and building relationships.
5. Keep your desk clean. Never go home with a messy desk. There's often a mental switch between being in a cluttered space and a clean one. Something as simple as making sure piles are picked up or organized can be the difference in my sanity for the day. (Thankfully, this is becoming more and more possible as we put more and more of our stuff on Canvas and I don't have nearly as many papers trying to drown my classroom!)
Hope these tips help to keep you sane!