I have been less "active" in the last 3 weeks than any time in the last 5 years. I went from working a job that had me on my feet all day, walking constantly, and lifting heavy items, to a job where I sit at a desk and have no need for serious physical expenditure. According to my Apple Watch, I've gone from getting 18-20,000 steps per day to 1-2,000 when I get home from work. After obsessively tracking my activity for 2 years, this simply will not do.
But I'm still extremely tired at the end of the day. I'm sleeping like a baby.
The best reason I can come up with is that while my body is not getting the exercise it used to get at work, my mind is being pushed more than it has in recent years. I wouldn't say my job was a cakewalk (far from it, actually) but I had been doing it for long enough that I knew how to handle just about any situation I was presented. In my new position, in having to learn a ton of brand new stuff quickly. I'm taking in a ton of new information every day.
I always knew that "hard work" came in many varieties, but I didn't fully appreciate how much energy working at a desk could take from you. I guess my big takeaway is that it's damn near impossible to know that someone else's job is easier than yours from the outside. Here's a spoiler, if someone is doing their job well and are making it look easy, they're probably working incredibly hard to make you think that.