Starting a business while working full time

How realistic it is to start a new business while working 8 to 5 full time? I'm starting to develop an online business, I have a good plan and in my opinion good chance to succeed.

My only problem right now is time and energy, as I don't want to quit my current job.

was anybody here able to achieve that? get enough income from side business while still working, until your side business makes more?