That is unless you agreed to a contract that forbids from holding multiple jobs.
40h/week paying the bills leaves plenty of free time to do something on the side when you don’t have kids / a family to take care of
Show of hands: How many others feel this way?
Personally, I can barely muster the energy to do much hacking after a normal job. The only way I've found to do work I've been known for is by blowing off my other responsibilities (sometimes to the annoyance of various people).
1: One needs downtime, and can't expect perfect utilization even after planning for rest.