Academia often fosters an environment where it feels like you have to work 18 hours days, just to keep up. Some old professor says 'it's part of the job'. Don't buy into it.
You are allowed time off. You are allowed interests outside of your research.
Take a break. Read a book. Spend time with friends. Spend an extra hour in bed. Relax. Recharge. After all, you can't pour from an empty cup.