Provided, that is, that the killing is painless. I take this to be a logical development from the proposal of two philosophers, as reported here and here, that after-birth abortion - what you and I know as infanticide - is sometimes justified, if the interests of the parents require it. Like the foetus, so the argument goes, a newborn baby is not yet a person (though it is a potential person); so it is not in a condition to be ascribed a right to life.
I avoid the knotty question of what the criteria of personhood are (self-consciousness, capacity to reason, having a life-plan etc), and merely go on what the two philosophers whose view is under discussion here say when they say:
We take 'person' to mean an individual who is capable of attributing to her own existence some (at least) basic value such that being deprived of this existence represents a loss to her. This means... that all the individuals who are not in the condition of attributing any value to their own existence are not persons.
I put it to you, friends, that someone who is fast asleep is incapable of attributing basic value to her own existence, and therefore a prime target for being killed, without that representing any loss to her. You might say that she will again be capable of valuing her own existence when she wakes up, and I do not deny this. But it only means that, once a person - before she went to sleep - she has ceased to be that since dropping off, and is now only a potential person until she wakes up again, if, of course, she ever does - since we can justifiably kill her (as being an actual non-person) if it serves our interests to do so.
Again, you might say that the definition of personhood builds in a notion of continuity and duration, such that a sleeping individual may be credited with the capacities which the same individual has while awake. But what a convenient allowance that is. For somewhere between a quarter and a third of most people's lives their capacities are not manifest while they sleep but, like them, dormant; and yet they are acknowledged still to have them. That could cover anything between 20 and 30 years, without personhood being snatched from their sleepy selves by others. No, let them be bumped off when there's something to be gained by doing so - that's what I say.
Or perhaps not, hey. The threshold of personhood is not some hard and fast line laid down by Nature. It is a conventional one that we use, for among other reasons, to draw a line of protection around members of our species. As things are, we draw this line at the point of birth and to include all of humankind. We can leave it like that, or else open the way to the slaughter of newborn babies. Take your pick. And, in fact, why not go further than newborns if that's the choice one makes? Is a two-month old capable of conceptualizing the value of her existence? Is someone under general anaesthetic? (I wonder, by the way, if these two philosophers have given any consideration to what methods of killing babies would be most suitable.)
If sleeping folk may be permitted years, even decades, as persons when they are plainly 'not in the condition of attributing any value to their own existence', we can permit newborns some little time to form a conception of themselves and of why they matter. Treat it just like giving them a short spell to awaken.