Increased pay. Long shifts tend to command a premium. Also if demand happens to be low and you manage to take a little nap, you're being effectively paid for your sleep time.
It is common to be scheduled a 24 hour shift - it is expected that you will find time to sleep but will be woken up to tend to an emergency. firefighers and EMTs work this way - sleep in the firehouse (or whereever) but if there is a 3am alarm jump and go to work. I know an ER doctor (in a very rural hospital) who also works this way, when there is nobody needing care he just finds a spare bed and sleeps until someone wakes him. Big hospitals might have enough overnight work to keep some doctor up all night but they still need some sort of backup system because a few nights a year things will be extra busy.