The_master_and_his_emissary
If you are interested in the differences in the right and the left brain and how these have shaped our world then this book by psychiatrist Iain McGilchrist is a fascinating read.
More about this in ‘Integrated’

Brain researcher Jill Bolte Taylor talks about her stroke insights

4temperaments4elements
Read more about the ‘Four Temperaments’

Heart Rate Variability – ‘HRV’
There has been research into HRV as a marker for health and disease over the last few decades. This has led to institutions such as HeartMath springing up delivering biofeedback technology to the public.
In balance (between the sympathetic and parasympathetic) the heart rate varies from beat to beat as depicted by the smooth sine wave.