Well, people in murder mysteries rarely act sensibly. For one thing, why wouldn't they all just stay together in one room after they realize a murderer is on the loose, they could take turns sleeping. It's not that much to do for three days until the boat returns - especially when your life is clearly in danger and someone has announced their intentions to murder you.

Don't forget that two of the four deaths at this point have been poison/drug-related. While Mr. Owen knows that he only has enough for one more murder, the other five people left don't, and have no reason to assume soparticularly since one of them is a doctor.

Yeah but somebody getting his skull split open, getting shot, etc., somebody would hear that.

If you've been up for 30-40 hours, most of which were spent in a state of nervous tension? Not a very heavy sleeper, just an exhausted one.
And remember - no one hears the fifth murder, because an injection of poison doesn't make any noise.

How heavy a sleeper would you have to be to not hear 3 other people getting murdered when you're all in the same room?

I sure as Hades can't do that now, and I'm a fortysomething year old in reasonable shape. No way can Cannon and Emily do that.
And besides, as they point out in the 70s version, the last one to fall asleep would have the rest at their mercy.