I suffer mainly in cars and buses/coaches. I'm happy to be sick in someone's car, or out of the window if I'm quick, but I will absolutely not risk it on public transport. I felt sick on an early train once, but we hit a signal failure and were sitting on the tracks for 20 minutes, so it subsided. But I was definitely ready to go to the toilet

And I assumed it would carry over to planes, so just took the 'safer' option for me and didn't go to sleep. I don't know why it only occurs in the first 3-4 hours after sleeping. But I'd rather be safe than covered in sick.