Janu Sirsasana (JAH-new shear-SHAH-suh-nuh) stretches the entire back of the body where all the major muscles are. This asana stretches the hamstring and gluteal muscles and is best done after standing asanas to remove fatigue from the legs. This asana also massages all the vital organs.
All forward extensions bring calmness to the mind. Because the heart is brought close to gravity the heart beats slower and calmer. This calmness is then received by the central nervous system and then brought to the brain. We are generally frontal body orientated: we are more conscious of the front of our body than the back body. Forward extensions bring our awareness to the back body and this creates a more reflective mind than an outward mind.
Avoid this asana is you have any disc-related conditions. Learn first to extend the spine rather than bend the spine downwards. Concave the midback to sustain the length in the lower portion of the spine. Women who are pregnant or menstruating should avoid going all the way down in this pose. Keeping the back concaved will sustain the space in their abdomen. Those with any stomach pain or cramps should avoid this pose. Sit higher on a folded blanket or a block if your hamstrings are tight.