You know what? I bet Atem is a deep sleeper
And when I say "deep sleeper" I don't mean "takes a few tries to wake him up", I mean "is often told it'd be easier to wake the dead"
This wasn't as much the case during his first life- though it was to some smaller degree- but once he comes back from the afterlife into a new body of his very own it's just one of those things that Happens TM
Sometimes it's actually pretty concerning, in the beginning Yugi panics alot because obviously he's afraid that his partner is dead again, he eventually starts keeping a compact mirror on the nightstand to hold under Atem's nose when he gets worried because ofcourse he breathes so lightly when he's asleep that Yugi can't really tell, it also makes it especially difficult to wake him up from nightmares- wich he has frequently- and let me tell you it's a PROCESS to do
....
But also, sometimes it's pretty fucking hilarious
Whenever he wakes up Atem is the kind of groggy that you would expect a zombie to be, body swaying, head flopping, eyes only occasionally opening, he looks like a two-year-old who's trying to avoid a nap every single time and it never stops being amusing, he also has zero motor skills when he first wakes up for atleast several minutes so if he makes the mistake of trying to do literally anything first thing in the morning he'll walk into walls, trip over thin air, stumble around like he's drunk, and that's only speaking of what's going on with his legs, the poor guy can't pick up anything without dropping it and that's assuming he can get a grip on it to begin with, but that's only mildly amusing, what's REALLY funny is what he says when he's not really awake
I wouldn't say he sleep-talks so much as I would say that he has times of being pseudo-awake, where maybe PART of him is awake but he's still mostly asleep and has no idea what's going on with anything but wants to try anyway, and the things that come out of his mouth during these times range from mildly amusing to practically rib-breakingly hilarious
The first time it happens, Yugi is the witness (ofcourse), he "wakes up" rather startled and insists that something is happening to Superman, now this immediately gives Yugi pause, as he knows with absolute fact that Atem doesn't even know who Superman is
At one point Kaiba startles him after Atem falls asleep in his office and is greeted with "Cats deserve more respect" before he just crashes back down again, as if that declaration was his sole purpose for "waking up" at all
He breifly wakes up around Joey once, looks him dead in the eyes and says "I'm not here and neither are you" before passing out again, Joey never stops finding that hilarious, he's still laughing about it fifty years later, his first task upon dieing and entering the afterlife is to find Atem and go "I'm not here and neither are you", it's the funniest thing that's ever happened to him
He startles awake once for no apparent reason, screams "BITCH!" at the top of his lungs, and crashes again, Yugi was left between nearly having a stroke and not being able to quit laughing
Sometimes he'll wake up and be juuuuuuuuuust aware enough to recognize some part of his surroundings- that someone is talking to him, for example, or that he's outside- but that only makes it to about a 1 on the "On a scale of 1-10 how awake are you now?" scale, that becomes obvious when he wakes up after falling asleep on a plane ride, stares at Tea` and asks "Where are we?" , she answers, he stares at her as if she's crazy and responds "No not there" and then just sort of.... falls asleep again
Not helping matters? He has some problems sleeping most of the time so he never gets enough sleep, meaning that he'll doze off every now and then when he kind of shouldn't, mostly when he's traveling somewhere (on a plane, a car ride, a train, etc) or when he finds himself in Kaiba's office for something (look it's a comfortable couch and the office is quiet and familiar and it just HAPPENS) so sometimes if someone who isn't part of his immediate social circle (and therefore in on the "secret") finds him in one of these positions it creates an awkward moment, once one of Kaiba's secretaries finds him asleep in the office and nudges him to tell him that Kaiba is running late, he jolts awake, slurs out "I can't let go of the earth it's spinning too fast" and then falls asleep again (no one ever said his ramblings made any sense)
There are plenty of instances out there and he doesn't remember ANY of them, when he DOES wake up and sees someone repressing a grin- or flat out just laughing at him- there's always a moment of "Oh shit what did I say this time?" with varying degrees of embarrassment to follow
At this point it's just basically a running joke between everybody, they start using his little mutterings as what's almost a code language, Kaiba will never admit to it but even he thinks it's funny and gets in on it sometimes (when he can disguise it as a brutal mocking instead of just having good-natured fun with everyone else that is), and even Atem himself starts finding it more funny than humiliating, really he's just happy to have provided something to make everyone happy- even if he never has any control over what it is