As a Wix Web Designer and Graphic Designer, I see a lot of people in my industry working super long hours. Here's my advice having successfully had my own small business for the past 16 years:
Make sure to take time off from work.
I find that if I work long hours (I definitely have enough work to work 24/7 it seems!), I am not as efficient. If I work 8 hour days and take 2 days off a week, I get a lot more done in less time. When I work more than 8 hours a day consistently, my design skills and decision making skills decrease substantially. So now I make sure to spend time outside in the sun and garden in between working. It's important!!
Also, make more realistic deadlines (pad them with extra time), and, if you are running behind, let the client know as soon as you can, and make sure to tell them when you can get it done. I always ask myself, "Is anyone going to die if this doesn't get done today?" and the answer is always no in the design world. ;) Why kill yourself? Just be realistic and communicate with your clients and they will understand.
So, stop and smell the flowers! You'll be more productive when you get back to your desk after a walk, some gardening, hanging out with friends and family, cooking good homemade meals, and other non-computer related activities. You're mind and your eyes and your designer soul will thank you!