What's it like sleeping in (on?) a submerged submarine?

I might be a little too claustrophobic to go on a ride myself, although I have never tried. And I feel immense respect towards people that do serve onboard submarines. With all my claustrophobia, I have this positive prejudice that it must be amazingly cosy to sleep onboard: feeling slight motions as the submarine moves, hearing the engine hum, having a "sleepover" with people you trust with your life...

Do any of you have any experience sleeping in (why do I want to say "on"...) a submarine? What's the actual feeling like? 😊