"The study suggests a significant association between stress and severity of acne papulopustulosa, especially in males. Increased acne severity associated with stress may result from factors other than sebum quantity. "
Yosipovitch, Gil et al. “Study of psychological stress, sebum production and acne vulgaris in adolescents.” Acta dermato-venereologica vol. 87,2 (2007): 135-9. doi:10.2340/00015555-0231
"Conclusions Patients with acne may experience worsening of the disease during examinations. Furthermore, changes in acne severity correlate highly with increasing stress, suggesting that emotional stress from external sources may have a significant influence on acne."
Chiu, Annie et al. “The response of skin disease to stress: changes in the severity of acne vulgaris as affected by examination stress.” Archives of dermatology vol. 139,7 (2003): 897-900. doi:10.1001/archderm.139.7.897