Doing Hot Yoga is like running 5 miles. You must eat, drink and prepare for this kind of exercise by following these safety tips.
Sweating profusely causes dehydration. In order to prevent this, you must drink at least one 16oz. bottle of water through the first 40 minutes of class. Classes are 75 minutes long, so you must have (and drink) two 16 oz. bottles of water during the course of the class. Signs of dehydration are nausea, dizziness and weakness.
Take your time during your trial week to acclimate to the conditions of the hot room by lying down, going at your own pace (like walking in a marathon), and avoid raising your head up too quickly.