Another teacher here.
I work part time (4 days) and this helps maintain a work-life balance.
The stressful part for me comes with the fact that you are always 'on-duty' in the job. Constantly thinking of what you are doing now...the next lesson....after lunch.... clubs at lunchtime.... afternoon lessons etc. And meetings after school and further clubs to do cut into the time I have left for planning or marking.
If you feel like rubbish, are unwell or struggling in any way with non-work things you still have to be at the top of your game. You can't have an 'off' day. Ever.
You are responsible for every child achieving the best they can, and it's almost asking the impossible to truly understand the best for each and every child in the class. So you do your best, but still think you could have done better as it's an intangible thing.
If the results aren't good, it's not the child that's looked at, it's the teacher.
And don't get me started on the question "Could they be bored?" when you tell a parent that their child doesn't listen well.
Luckily, I'm less stressed than normal at the moment as all my planning is in place (minus whatever tweaking needs doing to tailor it to the children) and on the computer along with differentiated resources we've built up or created. I'm quite efficient and don't waste time. I also have a smaller class than normal this year which really helps. I don't take much work home and don't have to spend too long on the holidays in my classroom or in the holidays. I also am in the lucky position to have a job in a school that has a 'good' catchment area and truly poor behaviour is not often a problem.
But the phone call to tell us we are going to be inspected is due any day.... so things may change. Lesson observations happen frequently anyway but I hate them. I'm a good teacher. I know I am. My class enjoy what we do and they get good results. But someone can stand there, watch what I'm doing and tell me how to improve. Meh.