While we think of erections as a response to sexual arousal, it’s important to know that many of the erections a person has over their lifetime will pop up as a result of something else entirely. Changes in blood flow, hormonal fluctuations, high levels of stress, anxiety, or nervousness, and even changes in ambient temperature can cause erections—other times, there’s no discernible cause whatsoever. Called “spontaneous” or “unprovoked” erections, they’re an ordinary (albeit sometimes conspicuous) physiological response.

A lot of these unprovoked erections—between three and five per night, on average—happen while their owners are sound asleep. They typically happen during the rapid eye movement (REM) sleep stage, a phase associated with vivid dreaming. Interestingly, however, these erections are not typically related to sexual arousal or dreams of a sexual nature, although they can co-occur.

https://www.treatmyocd.com/what-is-ocd/common-fears/worried-about-getting-unwanted-erections-youre-not-alone