Many of the successful people will stop for a minimum of one hour for lunch!
It isn't that they desire a break to eat elegantly or to goof around; it is because they know it is crucial for efficiency. If you frequently strain your eyes, they'll become sore and you'll get a headache.
The same holds true for your level of concentration. You must relax. You have to move your legs around, get your blood flowing, and receive a bit of sunshine. It'll open up your mind and rejuvenate your spirit.
- Murray Newlands