David described second series as a “wild” series… while primary is very grounding and introverted with a lot of forward-bending, second series is more energizing and extroverted with a lot of back-bending and opening of the spine. He said this is why when people start second series they notice changes in their energy levels, and especially trouble sleeping.
I wonder why back bending has this effect -- more blood flow and oxygen to the spinal cord stimulating the sympathetic nervous system? Or maybe compression of adrenal glands and stimulation of cortisol release? Any other ideas?