First of all. Sleep. And sleep well. Increasing sleep hours increases productivity and not the other way around. Sleep early (people are not supposed to work too late at night) and wake up after good-solid 8 hours of sleep. Then when you wake up, you will realize that you have amazing clarity and excellent productivity. Also, two hours before sleep abandon work and do something relaxing, such as listening to music or spending time with your significant other. Work or stressful activities before bedtime, disrupt your sleep and its quality. So, that leaves you about 24-(8+2) = 14 hours to work which are more than enough.
Devote at least one hour per day on average for exercise. Any sport, workout is better than nothing. Also, try walking. One hour of walking per day does wonders for your heart, lowers stress and you can still think about research while doing it (I do not advice this though). It also alleviates headaches (stress or work related). Also, mind what you eat. Keep your weight steady and do not eat too salty or fatty foods, that inhibit body and mind performance. Use a multivitamin every two days (after consulting your doctor). Also remember that a healthy body always performs better, including mind activities. Also maintaining a good, healthy appearance increases your chances for a fruitful social life, which will prevent you from overworking and overstressing yourself.
And just a reminder. You should not spend too much time on Stack Academia as well :-)