The most likely reason for acne cosmetica is not removing makeup at the end of the day, either completely or partially. With the exhaustion of show week, longer rehearsals and late nights, when I get home after a show, relaxing and getting into bed takes top priority. We’ve all done it - falling asleep with makeup on!
It’s really important to cleanse every evening, but especially to take makeup off, because your pores are more likely to get clogged that can lead to bacterial growth in the skin, which then causes spots and breakouts.