A survey released in May 2004 by the National Center for Complementary and Alternative Medicine focused on who used complementary and alternative medicine (CAM), what was used, and why it was used in the United States by adults age 18 years and over during 2002. According to this survey, Yoga was the 5th most commonly used CAM therapy (2.8%) in the United States during 2002. Yoga is considered a mind-body intervention that is used to reduce the health effects of generalized stress. Yoga is believed to calm the nervous system and balance the body, mind, and spirit. It is thought by its practitioners to prevent specific diseases and maladies by keeping the energy meridians open and life energy (Prana) flowing. Yoga is usually performed in classes, sessions are conducted at least once a week and for approximately 45 minutes. Yoga has been used to lower blood pressure, reduce stress, and improve coordination, flexibility, concentration, sleep, and digestion.