A prospective longitudinal pilot study of tai chi for young adults, carried out over 18 weeks, found that their subjective health increased. Stress, measured both subjectively (by questionnaire) and objectively (by measurement of salivary cortisol levels) was also found to decrease during tai chi practice. (Mind/body techniques for physiological and psychological stress reduction: Stress management via Tai Chi training – a pilot study. Med Sci Monit. 2007 Nov;13(11):CR488-497).