Make it so sleeping is like hunger or thirst, but instead of causing damage it drains your mana.
Otherwise you can go for days and days without sleeping if you aren't taking damage.
20-24 hours and you need to sleep for say, 3 hours or something. No one could keep running like that without sleep.
Or it could make be dependent on int or wis. the higher the stat the less sleep you need.
and introduce no-doze trucker pills!
Honestly, I think managing sleep would be a little overkill and would detract from the fun of the game.
While I can see the benefits from having something like this implemented, I don't see it as practical in the game. There are so many drains on your mana as is we don't need more. I have had battles with people that have literally taken hours to get through, and they end in a stalemate. Now factor in cabal warfare. Can you imagine being drained, and sleep drained? You would never heal mana ever
Mana potions ftw!
If it existed then CON would be the driving stat not int or wis. Mana potions do exist and I know where to buy them, the question is are they cost effective?
Ofcourse they exist, and at more then once source, and in more then one form.
If this change was implemented then Id say hell yeah they were worth it 
There are certain levels of realism that aiming for is a mistake - I think this is one of them. I do not see any way in which this would make playing the game more fun - quite the opposite, I think this would be mostly an annoyance, in situations ranging from cabal war to extended RP sessions to ranking. While Fallout benefited from the addition of the Hardcore mode for New Vegas, FL is not Fallout - we're allowed a greater suspension of disbelief.
There are certain levels of realism that aiming for is a mistake
Well said.
Gameplay > Realism