Working from home is exhausting when you can't leave. There becomes no difference between 8pm on a Saturday and 10am on a Tuesday. If you're awake, you may as well work. Do you want to take a break? You can walk maybe 5 feet. You're walking around significantly less, and changing gears less, so everything becomes some really weird blur. Any distinction between work and not work totally ends. Yeah, you can set boundaries, but at some point they stop feeling as if they matter. You're going to be on the same couch or seat. There's not much to do. May as well do some work, especially since you probably have Teams and Outlook on every screen you loonat and can see it's buzzing with a question.
It's not physically exhausting, it's mentally.