Yoga is a "combination of breathing exercises, physical postures, and meditation, practiced for over 5,000 years, calms the nervous system and balances body, mind, and spirit. It is thought to prevent specific diseases and maladies by keeping the energy meridians (see acupuncture) open and life energy (qi) flowing. 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. It has also been used as supplementary therapy for such diverse conditions as cancer, diabetes, asthma, and AIDS."