How does Darth Vader sleep with his suit?

In 'Rogue One' we see him suspended in a bacta tank, entirely out of his suit. Presumably this is as close to restful sleep as he gets. Outside of that, there's his meditation chamber featured in 'The Empire Strikes Back' which allows him to at least remove the helmet, and as the name suggests; meditate.
Keep in mind though that Vader is never "comfortable" or "restful". He's a raging inferno of constant agony and hatred. He may not ever truly "sleep".

The Legends canon novel Shadows of the Empire loosely following the story of the game of the same name describes a spherical "hyperbaric medical chamber" within Vader's residence on Imperial Center as it's referred to at the time which provides a "supermedicated and oxygenated field" in which Vader is able to survive without his armour.

This may be identical to or a larger version of a chamber we see on the Executor a couple of times in Empire Strikes Back where Vader tends to sit when not offing subordinates or taking calls from the Emperor. In one of these scenes just before the Battle of Hoth General Veers sees Vader's bare head from behind just before his helmet is lowered into place.

in the Star wars movies we can see that Darth Vader have his own personal chamber made to assist him with the equipment /suit he wears as well as, for resting/meditation purposes. and also that Jedi/Sith, especially those trained to use the force having mastered it, don't need the same sleep requirements regular People need. They can "sleep" sitting straight up, on top of harder elements and they can do energy from the force to help them stay awake. The expanded universe, description books, gives more examples of Jedi/Sith sleeping or measures they take.