I started doing Ashtanga Mysore style yoga a month and a half ago, and I'm bursting with energy throughout the day. I'm saying this as someone who is 41, has spinal degeneration and is a cancer survivor, and I feel years younger after practicing. It's a more traditional style that is done very early in the morning, and has an exact sequence which is great for those who like structure. The expectation is that you practice every day except Saturday. It's pretty demanding physically, but can be adapted for those who are less fit and flexible. A lot of thought has been put into the design of the sequence, in that each posture and movement is preparation for another movement later in the sequence, so it leads you step by step to do things with your body you wouldn't have guessed you were capable of. The sequence emphasizes both strength and flexibility, and my body is already taking a much nicer shape after only a short period.