If I was to pickpocket a NPC clothes off while they sleep, will they

A§) Put some new clothes on, soon enough.
B§) Will go naked, forever.

And if it's B.
I am a bit disappointed and it will become a very naked world in the end if there is no "brain function" for NPC to get dressed again.
Because I remember Bethesda talk so fondly about Oblivion, how the NPC has routines and they would eat, poo, do all that they needed and pick up things they saw.

But if it's A!
I'm happy, nothing to complain about.

if B comes true then the biggest thing for the developers to worry about in 1.6 is re polish the NPC system if it's A think of the possibilities

They never put their clothes back on, trust me, I have towns protected by naked guards, and it scares me.

Sounds fun

Did you steal EVERYTHING from them? Because it seems if they have something in their inventory they'll never respawn their items. If you take it all, they seem to restock after a time.

If you take all the pieces of their outfit, their clothing will reappear once the cell unloads. You can tell if it's part of the outfit if the NPC has it equipped -- not equipped, not part of the outfit.

You cannot take the equipment from essential NPCs.

Depends. I've been able to steal weapons and certain pieces of clothing off on essentials.

I think the essential thing only applies to clothes, not weapons.

So if I steal all clothes, it will come back to them? Or? Did I have to take _EVERYTHING_!

Yes, you only need to take their clothing.

Essential NPCs can be pickpocketed for their weapons and shields, but no other worn objects.

It's a bit of a strange one, and it always surprises me when I return to Breezehome to find Lydia sitting in there in her golden undies. When I married her I was able to buy her default armour from her, which I did, but I made sure she has something else for when we're Questing together. When she's "relaxing" at home she removes the armour I gave her, (though she still somehow manages to hook Wuuthrad into something on her back!!)

When you piclpocket equipped weapons they can pick them back up again if you leave it at thier feet. Not to sure about clothes though.