I have the very strong belief that I require breaks, just as I require food and sleep -- and if I don't take them, my brain will take them for me, and that's when I end up with awful days staring at a piece of paper unable to do anything.
If it's a chronic problem of never making use of the time I have, then that's something I gotta take steps to change...... but if I just have a few days off and don't get anything I wanted done? I have to remind myself that I must've really needed a break. I used to have a habit of, every time I overslept and was really mad at myself, I would say a quick prayer where I thanked God for the extra sleep that I must've needed. It's just reframing it, looking at it differently -- because there's no way for me to change the fact that I took a break, I might as well let myself be grateful for what I did get from it. Plus it's important for me to fight the part of my brain that wants to punish me for resting, because that part of my brain is very strong.
If you (or your brain) needed that time off to veg out and recharge, and you come back and hit the ground running, then imo, it isn't wasted time. On the other hand, if you beat yourself up over it, push yourself to burnout levels because you're mad at yourself for taking a 4 day vacation from both art and school, and don't let yourself benefit from a restful vacation, that's when you're letting it go to waste!