According to a study in the Journal Psychological Science, the posture you adopt measurably impacts on hormone levels. People were placed into ‘high power’ or ‘low power’ postures for 1 minute and then had their saliva tested for testosterone and cortisol. Significant changes were found in hormone levels. They also tested the testosterone levels of a reporter for a Time magazine article. He was asked to stand in the ‘hawk’ position (leaning forward / fists on desk), and feet-on-the-desk "power poses", and they reported that his testosterone levels doubled in the short amount of time spent in the power positions.
The study suggests that posture affects emotions as much as emotions affect posture. And the theory is that training posture strengthens emotional as well as physical health. So, hold your head up!