Aspect of the Moon (an eldritch invocation for the Pact of the Tome warlock) grants this benefit:
You no longer need to sleep and can’t be forced to sleep by any means.
This seems to leave open the option to sleep if the warlock wants to. Normally characters (or real people) can’t just decide to fall asleep, though, they have to be sleepy/tired. But presumably the warlock does not get this kind of sleepy, ever, at least not so much that they’d need to sleep.
If a warlock wants to go to sleep for some reason, how does this work? Can they just choose to fall asleep at any time (if it’s quiet enough etc), despite not needing to sleep?