Stress causes worsening of acne in two ways. First, by stimulating adrenal glands to produce more hormones.
In stressful situations the hypothalamus releases a chemical called corticotropin -releasing hormone (CRH). The oil glands of skin are known to produce both CRH and CRH receptors. So, when the CRH receptors come in contact with extra CRH, it induced sebum production by oil glands which ultimately resulted in the exacerbation of acne. In other words, stress causes your brain to release chemicals that increase the oil production which then in turn begin to clog the pore, allowing the natural P. acne bacteria in your skin to begin to overproduce.
Secondly, stress causes or worsens acne by slowing down the healing process.
It has also been established that psychological stress can decrease the wound healing capacity of immune systems by up to 40%. This factor doubles the impact of stress on acne.