Comment by marginalia_nu

If you aren't able to challenge yourself at your job through your work, I'd suggest you talk to your boss and say you're feeling you could do more and see if you can move to another role, or look for a new job entirely.

It's really hard to do much productive thinking outside of a full time job. Trying will make you burn out pretty hard. Use that time to fullfill the needs your job can't. Go to the gym, socialize, etc.