If you feel healthy and you're getting
enough sleep then there's no problem. Working nights is fine -- the thing that drives people nutty is
rotating shifts, e.g. switching between a night and day schedule too often.
It all depends on what you enjoy, and that's the bottom line. Bill Gates wanted to have his own company more than anything else; he wasn't doing it to make money "so he could do what he really wanted". The #1 thing he wanted to do was run his own software company, and you can see that despite becoming a billionaire long ago he didn't stop. Neither did Steve Jobs. If you're doing what you love then you're doing the right thing.
That said, humans need variety, so when you want to do something else, don't make yourself work more. Making yourself work more is only for people who need discipline and aren't putting in a decent number of hours to start, not for people working overtime to work even more. Bill and Steve had a LOT of variety at their jobs and plenty of social functions (including company social functions).
Think of it this way -- you're making a huge investment of your time and energy, but you're going to BLOW IT ALL if you burn out. You'll get like 80% done and then it will just sit there forever and you won't want to look at it.
So, make sure you don't burn out.
It's a bit like overtraining when you work out; except severe burnout isn't like fatigue that gets better in a couple weeks, it's like needing hip replacement.