Recent studies have shown that frowning, grimacing, and other negative facial expressions send a signal to your brain that whatever you are doing is difficult. So your body responds by releasing cortisol, which raises your stress levels.
Stress begets more stress...begets more stress...and in no time, you're super stressed!
INSTEAD- MAKE YOURSELF SMILE. YOU'LL FEEL LESS STRESS EVEN IF NOTHING ELSE ABOUT THE SITUATION CHANGES.
And there's a bonus: When you smile, other people feel less stress, too. Which, of course, will reduce your stress levels. So kill two stresses with one smile.