Stress.  

Although stress itself does not cause acne, it can lead to hormonal changes which can affect the amount of sebum oil produced by the body. When you become tense, your adrenal glands go to work, flooding your bloodstream with the hormone cortisol. This triggers the sweat glands in your face to produce more oil. When your sebaceous glands go into high gear, there’s a higher probability that this excess oil will mix with dead skin cells and clog your pores, trapping bacteria inside. The result? More acne.

Below are a few tips for avoiding stress related acne outbreaks.

You can't eliminate stress from your life altogether— it's part of being human. But, you can minimize stress by leading a healthy lifestyle. A balanced diet and at least eight hours of sleep every night will help you build a stronger physical foundation; if you're well fed and well rested, you're less likely to feel irritated by the events of your day. In addition, try to get some exercise every day, even if it's just a walk around the block at lunchtime. Take time out of every day to relax — read a book, take a bath, practice yoga, or do whatever makes you feel happy and calm. It's an important step towards overall good health, and therefore the health of your skin.