When I was in music school, many of my peers took pride in how much they practiced. The goal was to spend as many hours per day as possible in the practice room, and heaven forbid you miss a day. Sheer volume of hours accumulated was king.
When I was in music school, many of my peers also struggled with overuse injuries, such as carpal tunnel syndrome, as well as a nagging lack of satisfaction with their progress.
Are these two things related? Absolutely.
Practice is how we get better. But when we work toward acquiring a new musical skill, we have to make sure we're ingraining the right things: ease of technique, healthy body position, and confident performance style. Frustration, annoyance, and inattention are all you'll learn when you practice for the sake of racking up your hours.
Your practice sessions should be inquisitive and illuminating. If you find yourself losing focus, find a way to bring yourself back. There are so many ways to approach the same subject that there's no reason to be bored in the practice room. If you're not sure where to begin, start with some opposite practice.
One of the most helpful and rejuvenating techniques I use is to skip a day on purpose. I use this day of rest to focus on aspects of my musicianship that sometimes take a backseat: I might spend some time researching the history of a piece I'm working on or looking up definitions of unfamiliar terms in my sheet music. I might also delve into diagramming the form and basic chord progression of the pieces or etudes I have in my rotation. These are all things that can feel less important than spending a certain number of hours with your hands on your instrument, but they are skills that differentiate the great musicians from the rest.
Taking a day off also gives you a chance to want to play your instrument. If you're in an intense program of musical study, whether through a private teacher or in a conservatory, it can be easy to lose sight of how much of a gift it is to be able to play music in the first place. It can start to feel like an imposition or a chore. Taking one day off gives you a chance to miss your instrument a little and remember the value of what you do.
So one day a week, leave your instrument in the case. It'll be OK, I promise.