“Smiling broadly for two minutes and grimacing for two minutes have been shown to reliably alter mood”
This quote is from http://www.emotionalhealth.co.uk/smile.php It is a very short article based on D.N. McIntoch, R.B. Zajonc, S. Petrus, and S.W. Emerick in Cognition and Emotion, March 1997 “Facial Movement, Breathing, Temperature, and Affect: Implications of the Vascular Theory of Emotional Efference” showing that full-faced smiling alters cerebral blood flow and lowers cerebral vascular temperature, which they believe is the reason behind the feeling of well-being that smiling produces.|
Moreover, research by the French Physiologist Dr Israel Waynbaum indicates that the act of smiling induces chemical changes in the brain, which, he believes, are responsible for altered mood. Subsequent research has supported this.