Yes, the room is intentionally heated to about 90-95 degrees and in some studios, even hotter (Bikram studios will be around 105). While it is true that the heat is not necessary to practice yoga, it is absolutely safe and, in fact, has many benefits.*
Here are a few examples:
All that said, the heat will effect people in varying ways and caution should always be exercised. The heated room is a tool and not something you should have to survive against. Always listen to your body and take care of yourself. Proper hydration is essential before, during, and after class. As the sweat rate increases in a hot yoga class, body water loss increases, and without adequate fluid replacement, the body’s ability to dissipate heat is compromised. Electrolyte supplements like Ultima Replenisher are also helpful, especially when beginning a hot yoga practice and acclimatization process.
One final note, I have found that reminding myself of the benefits that the heated room brings during a difficult part of class completely shifts my energy and I am able to push though those layers of self doubt. It is my hope that this information also helps you in your personal practice.
Thanks for reading!
*Practicing yoga in a heated environment is contraindicated for people with Multiple Sclerosis. Always consult your physician before beginning any exercise regimen.
(ref. "Physiological Concerns while Exercising in the Heat " by Leslie S. Funk www.wellness-design.com )