Objects at rest tend to stay at rest, and objects in motion tend to stay in motion. That's inertia as we learn it in basic physics classes.
To divert from real science into personal practicality, if you have something that you HAVE to do, it is easier to muster up the energy than if you don't really have to do it, or if you can put it off until later without much trouble.
I had only recently recovered from a terrible cold, and I ended up with another one to start the week. I'm guessing that it came from the new preschool student in the house, who did have both a fever and a stuffy nose over the weekend. I don't think I had a high fever like The Boy did, because I never bothered to take my temperature, but I was definitely knocked on my behind from Monday afternoon through yesterday evening.
Fortunately, the weather cooperated with me and kept me home from Tuesday night's rehearsal and gave me a pretty easy out from last night's travelling music lessons, which I might have been able to handle if they were all close by, but they're far away with quite a bit of travel time in between each one. I have to make them up this weekend, but I'll manage. And since I am making them up, I won't lose the money.
I still had to get up to take The Boy to preschool on Tuesday, because he was feeling perfectly fine by then, but taking him to school is a net gain for me in the napping department, even with having to leave the house twice, because it meant that he was tired enough to nap with me.
I wonder, if I were working full-time, how many of these days would have been "sick days" for me. Maybe Tuesday, but probably I would have kept on trucking and gone to work, because once I'm moving, unless it's REALLY bad, I can at least function. It's hard to get up out of bed when you're sick, though. Your whole body feels heavy and slow.
Objects at rest REALLY want to stay at rest.