Lucas Blackstone said:
So players wouldn't be instant killed while they slept? That doesn't sound very fun to me.
That's up to the DM, but I would be inclined to accuse a DM who does this of being a poor DM. Of course, certain mitigating circumstances may apply...
In other words, I don't think most good DMs would arbitrarily kill off their players in their sleep.
I also don't think that most anyone in a society where being rich, powerful, or famous means you are likely to be assassinated in your sleep would actually sleep unprotected.
Sure, catching a farmer or a tailor in their sleep could be easy. But merchants, nobility, priests, leaders of the city watch, politicians, and adventurers would probably always sleep with some protection, such as having buddies or trusted guards to watch over them, or having guard dogs, or having magical protection such as an Alarm spell (haven't fully memorized all of the new PHB, but surely there is something equivalent to the Alarm spell from previous editions?)
In any case, it seems really weird to be able to whack away at a sleeping person's neck with a big old sword or axe, and end up only scratching their skin.
I say, if you catch them sleeping, or otherwise similarly helpless, you can kill them. And you can do that to player characters too, though as a DM, you should consider a different approach, unless you really have a good reason.