There are a dizzying number of styles and approaches to yoga these days. Some involve resting in simple supported postures in quiet, candlelit rooms. Others push students to the edge of their physical capacity or are done to the beat of loud, rhythmic music. There is so much variety that describing them all is impossible.
The bottom line... they work. Put simply, you feel better when you do yoga. The question is, why? Better yet, how does yoga work?
As you've probably heard, one reason asana leaves you feeling so good is that it activates your parasympathetic nervous system, thanks to two elements that almost all asana practices have in common—the lengthening and strengthening of musculature and calm, even breathing. The parasympathetic is the part of your nervous system that slows you down—it's responsible for telling your muscles to relax, improving your digestion and assimilation, boosting immunity, and helping you sleep better. It also normalizes your blood pressure and lowers your heart rate.
What else is helping you feel and live better? The answer is life force or Pranic energy. Almost all styles of yoga increase the flow of prana, or life force, in your body. Yoga, like the science of acupuncture, or tai chi and qi gong, is based on prana (referred to as chi in the Chinese arts and sciences). Asana (postures) is described as the foundation for hatha's deeper practices because it is so accessible and helps to free life force: The process of holding a pose—while "breathing through it"—dissolves pranic blockages. Different postures unlock prana in different ways.
A key reasons you feel better after class is that the practice has helped move your life force in a way that is more balanced, complete, or suited to your particular mental and physical needs and you activate your parasympathetic nervous system.