Found this study on Pubmed...

Optimism, positive affectivity, and salivary cortisol.
Lai JC, Evans PD, Ng SH, Chong AM, Siu OT, Chan CL, Ho SM, Ho RT, Chan P, Chan CC.

City University of Hong Kong, Hong Kong, People's Republic of China.

OBJECTIVES: Research on stress and salivary cortisol has focused almost exclusively on the effects of negative psychological conditions or emotional states. Little attention has been drawn to the impact associated with positive psychological conditions, which have been shown recently to have significant influences on neuroendocrine regulation. The aim of this study is to examine the impact of optimism and positive affect on salivary cortisol with the effects of their negative counterparts controlled for. DESIGN: Optimism and pessimism, and positive and negative affectivity were studied in relation to the diurnal rhythm of salivary cortisol in a group of 80 Hong Kong Chinese, who provided six saliva samples over the course of a day on two consecutive days. The separate effects of optimism and positive affect on two dynamic components of cortisol secretion, awakening response, and diurnal decline were examined. METHODS: Optimism and pessimism were measured using the Chinese version of the revised Life Orientation Test while generalized affects and mood states were assessed by the Chinese Affect Scale. An enzyme-linked immunoabsorbent assay kit (EIA) developed for use in saliva was adopted for the biochemical analysis of cortisol. Testing of major group differences associated with positive psychological conditions was carried out using two-way (group by saliva collection time) ANOVAs for repeated measures with negative psychological conditions and mood states as covariates. RESULTS: Participants having higher optimism scores exhibited less cortisol secretion in the awakening period when the effect of pessimism and mood were controlled. This effect was more apparent in men than in women who had higher cortisol levels in the awakening period. Optimism did not have similar effect on cortisol levels during the underlying period of diurnal decline. On the other hand, higher generalized positive affect was associated with lower cortisol levels during the underlying period of diurnal decline after the effects of negative affect and mood states had been controlled. Generalized positive affect did not significantly influence cortisol secretion during the awakening period. CONCLUSIONS: These findings suggest that positive psychological resources including optimism and generalized positive affect had higher impact on cortisol secretion than their negative counterparts, and point to the need for increased attention to the potential contribution of positive mental states to well-being.

PMID: 16238860 [PubMed - indexed for MEDLINE]

Another reason to keep into being the best you can be and getting in positive states of mind!