Try not to eat for at least 3 to 4 hours before a class as you might feel bloated and uncomfortable due to all the twists, stretches, breathing etc. that is done in a class. You can eat a piece of fruit plus minus 2 hours before. Take your last big drink of water 1 hour before class. Also try to wait 1 hour after your yoga class before you drink water, as the practise of yoga allows the energy (prana) to rise and if you drink too soon after class you suppress that energy.