I had the exact same situation, American living in Europe (CET) for 6 years. It was fine most of the time, but became exhausting after a while. A few things I learned:
- Set boundaries: can you ONLY do late night calls 3 days a week?
- Don't work mornings: my best times where when I'd go for a hike, do a long lunch with my wife, etc. during the typical work day, so I was fresh for my evening work time
- Shift your sleep schedule (if you can). Nothing wrong with getting up at 10:00 if you don't start work til 2.
- Find a social circle that fits this: in SEA there's probably loads of digital nomads and remote workers who are doing a similar thing. Connect with them and see how they're managing.
Either way, sounds like a fantastic experience and opportunity...definitely don't be scared by the timezone difference. You'll find a way to make it work.
I've 100% been in your shoes but gotta say, if you don't have customers you don't have a business, you have a hobby.
I would stop all development work on the product and sign up 10 customers. See what that process is like, see if you like it, hear their objections. You'll find a way to take sales calls during the work day (I used to from my car at lunchtime).
If you get 10 paying customers, why not 100, why not 1,000.
Can you automate the signup process or do a self-service demo somehow? I've done pre-recorded videos en lieu of sales calls for a time and it worked ok.
Generally though, I would NOT just quit your job if you have zero paying customers. Ease into it. Sounds like you've been burning the candle at both ends for a while, and I have mad respect for that, but probably best to keep doing it a bit longer til you have some sense for what revenue of the business could really look like. Especially if you have kids and a family to support.