This was reported today - Smiling Facilitates Stress Recovery
The results of the study suggest that smiling may actually influence our physical state: compared to participants who held neutral facial expressions, participants who were instructed to smile, and in particular those with Duchenne (genuine) smiles, had lower heart rate levels after recovery from the stressful activities. The participants who held chopsticks in a manner that forced them to smile, but were not explicitly told to smile as part of the training, also reported a smaller decrease in positive affect compared to those who held neutral facial expressions.
These findings show that smiling during brief stressors can help to reduce the intensity of the body’s stress response, regardless of whether a person actually feels happy.
It makes me think of the work that Marcora has done looking at how facial expression impacts on fatigue in his psychobiological model of fatigue. His study here showed how the onset of frowning indicated the point at which things were starting to feel hard, but in discussions by email we chatted about the possibility that things could work the other way - smiling might make everything feel easier and ward off fatigue.