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?
Why this matters: Apart from role playing or quest/plot related reasons, there is at least one mechanical reason to sleep: Dream spell allows a lengthy conversation between two creatures on the same plane, but the target of the spell must be asleep. A warlock with the Aspect of the Moon would need to know to go to sleep at the right time, but this can be easily arranged via Sending spell or a mundane letter or a pre-agreed time.
Best Answer
You can sleep if you want.
“Does not need” is not the same thing as “cannot”. This is evident from the feature’s use of both phrases.
You cannot be forced to sleep. This clearly means that no external force can cause you to be put to sleep.
In contrast, you do not need sleep, but this does not restrict your ability to do so. It simply means that you do not suffer any negative complications from insomnia.
As a DM, I generally just let my players sleep when they want, unless there is some extenuating circumstance preventing it, such as being in a rainstorm without shelter, or they just woke up from a long rest.