I was working with a client recently who was concerned about taking a planned vacation for fear they might disrupt "momentum" in their writing. We discussed strategies for staying connected, of course, but I mostly encouraged taking the vacation without reservation - rest when you've scheduled rest, no matter what the work is like.
I believe there are three main types of rest:
Collapse - The rest that happens when you drive forward and forward until your body literally shuts off. Think falling asleep on the couch when you thought you were reading, or when you have to take a week of couch time because your sniffles that you ignored turned into a massive sinus infection. You might feel better physically, but not restored.
Distracted Rest - This kind of rest happens when you take a break (pre-scheduled, accidental, forced, whatever) but your mind doesn't break with you. You could feel guilty for not doing enough to "earn" a break, or you could be stuck somewhere without your materials and feel bad for not working when you could have. Maybe you're out somewhere with family/friends/partners and you want to be with them but also, your work is on your mind. This kind of "rest" feels frustrating (maybe also for the people you're with!)
Actual Rest - This kind of rest happens when your brain and body are on the same page. You might leave your phone at home, maybe you read books for pleasure, maybe you just take a walk around the block with your favorite podcast. But, you feel rejuvenated afterwards because you let yourself rest, body and brain.
If it isn't clear, I think you should aim for actual rest whenever you can. Sometimes, you can't avoid collapse - sickness, deadlines, life itself can all make us need to close up shop physically. But I encourage my clients whenever I can (and try hard to remind myself) that we can rest and recharge effectively without taking a week-long phone free vacation. It can be as simple as reframing that time as "rest" and not "time not working."
I often hear clients tell me that they are using a night off or a nap as a "reward" for hitting a milestone, and I get the impulse to use these chances away from work as motivation to make sure the work gets completed. But the message that reinforces is that we are only "allowed" to rest or relax when the work is completed, not simply because rest and lives away from academia make us healthier and more balanced. We don't earn rest - we need it. And if when we do get a chance to take a break, so many of us stop ourselves from fully enjoying it by feeling guilty, or otherwise staying hooked into the work and not into the fun activity.
So if you have something scheduled and you feel behind? I will almost always advise you to go anyway, and do your best to fully enjoy it. Put your phone away, let your emails sit until morning and be present where you are. Even if your work isn't done, sometimes the time away can be recharging, and you'll attack the work the next morning with a renewed energy level and clarity. A healthy balance between rest and work can help to keep you healthier and happier. But, it even makes the work better, supporting a sustainable pace rather than a cycle of "grind and collapse" ruled by deadlines. So rest up, work better, feel better!