Definitely takes some discipline. I’ve found the stress more bearable as time goes on, but it’s definitely a thing, and having a demanding day job can make it worse. Taking a month off here and there from side work can go a long way, though, as can taking a week off the day job to finish a side project ;)
It was fun to work on something outside my day job but it required some judiciousness to ensure I could meet my set time limits.
I found taking on bigger projects + a demanding day job pushed me to burnout pretty quickly.