In his 2011 TED Talk, Ron Gutman referred to recent research about smiling.  Apparently one smile creates the same amount of brain stimulation as eating 2000 chocolate bars!  It is also healthier as it reduces the levels of stress hormones such as cortisol, adrenaline and dopamine while increasing the level of mood enhancing hormones like endorphins - basically it makes you feel better. You can view the whole talk here.