Well, there are several reasons!
Much of asana practice is about opening your heart and the action of rolling to your right continues this opening and freeing from pressure. Since your heart is on the left side of your body, when you roll to your right side, your heart remains above your organs, physically exerts less weight on your heart. The heart remains open and free of pressure. Resting on the right side allows your natural blood pressure to reach it’s potential homeostasis.
Traditionally, in India, it is encouraged to enter a room with your right foot. Also, in many parts of the world especially the east, we extend our right hand to greet one another. The right side represents the east and so rolling towards the right, the east, or the rising sun is symbolic of asking for blessings of grace and bliss.
Rolling to your right keeps your ida nadi (one of your main channels of prana, or life force, which corresponds to cooling energy) active and helps to keep your body cool and calm as you come up to sitting. Lying on your right side helps keep your left nostril, which is on top, open. This allows you to continue to breathe through your left nostril and continue the relaxing effects of lying on your right side (more details on nadis below*).
*Our being is considered to consist of much more than just our physical and visible body. Our body contains over 72.000 nadis, or energy channels, through which our Prana, or Life Force, flows. These nadis cannot be seen with the physical eye. The practice of yoga aims at releasing energetic blockages from the nadis, allowing prana to flow unimpeded around the body. There are three main nadis in the body a) the Sushumna in the centre of the spine b) the Ida nadi on the left side of the spine, c) the Pingala nadi on the right side of the spine.
The Ida nadi on the left side of the body, is related to our femininity. The feminine side of the body has cooling, calming, Yin energy. It is connected to the moon. The left side of our brain is creativity.
The Pingala nadi on the right side of the body, is related to our masculinity. The masculine side of the body has heating, energizing, Yang energy. It is connected to the sun. The right side of our brain is analytical.
The Sanskrit word for sun is *Ha* and that for moon is *Tha* – Hence *Hatha Yoga* The balancing of the right and the left side of our being.
The practical application of this is simple, yet profound. When you feel sluggish, tired and without energy, you can stimulate the right side of your body to energize yourself. When you are feeling overheated, irritated and spun-up, you can stimulate the left side of the body to gain a sense of calmness and stability.
Rolling to your left side in pregnancy:
Rolling to the left side during pregnancy (instead of right side) can help improve circulation, giving nutrient-packed blood an easier route from the heart to the placenta to nourish the baby.
Lying on the left side also relieves the pressure of the enlarged uterus from compressing the vena cava, which carries oxygenated blood to the baby; it also relieves the pressure on top of the liver, which process nutrients and detoxification of non-nutrients.
If you have any other theories, I would be interested to hear them 🙂 Namaste.