Studies have shown that the yogic practice of unilateral nostril breathing modifies the central nervous system. Utilized correctly, this exercise can alter mental clarity/focus and the body’s alertness as controlled by the autonomic nervous system. Depending on whether you use the right/left nostril or breathe unilaterally through alternating nostrils, the benefits can be lowered blood pressure, decreased anxiety, increased cognitive ability and decreased heart rate variability. There are quantifiable effects on the autonomic nervous system particularly in the eye where decreases in intraocular pressure are measurable.
Check out these links to read more about the positive benefits of unilateral nostril breathing.
http://home.gwi.net/~erichard/cerebr.htm
http://www.healthandyoga.com/html/research_papers/btp/om.asp
http://www.cnsspectrums.com/aspx/articledetail.aspx?articleid=1163
http://www.yoga-in-daily-life.org/articles/show.asp?id=20030716201
http://www.healthandyoga.com/html/research_papers/pig/om.asp