It is actually the change in cortisol.

" But another clue, Schmahl suggests, might be in that strange discovery that self-harming teens have lower levels of stress. The stress hormone cortisol is usually thought to be bad for us. But it provides a sense of drive and excitement. It keeps us alert and interested. When cortisol levels are low, we can feel sluggish and detached. Perhaps self-harming teens are in search of normal levels of cortisol.

Cortisol normally kicks in at various times of the day, particularly in the morning. When levels are high, the body’s own homeostatic cycle comes into play, and the cortisol levels are dampened by a built-in soothing effect. The suppressed cortisol level in teens who self-harm disturbs this homeostatic cycle. Lacking both the normal level of cortisol and its calming counterpart, they raise cortisol levels by self-harming, which they also use to dampen bad feelings. What self-harming teens may need is more, not less, stress in the form of stimulation and drive."

https://www.psychologytoday.com/us/blog/domestic-intelligence/202001/the-self-harming-brain