This doesn't seem that surprising to me, or at least I expect we should also see an increase on Saturday. People like to do harder work, like lawn projects on the weekend, and things like drinking that can affect the heart.
So it wasn’t so much about dreading the coming week but being still from the previous week and being sad about what they actually accomplished (or didn’t).
I suppose it’s a nice problem to have but still uncomfortable. Congratulations on your retirement and hope you get to figure it out and find a new mountain to climb.