Presumably it's supposed to be the case that he has some level of conscience regarding
all the terrible things he's done (e.g. spawning offspring on a multitude of worlds, only then to murder them, and possibly his later essential slavery of Mantis)
leading to serious insomnia.
How that timeline is supposed to interact with Mantis's lifetime is another matter. But it could be that she could be very long lived, or that there have been others in the past. Or it's supposed to be that his insomnia is relatively recent.
I originally wrote "presumably". Actually, there's direct evidence.
MANTIS: He lies awake at night - thinking about his progeny.