That is an odd perspective. That's like saying that the damage from a twisted ankle is permanent, since you'll never get back the two days of lost activity.
From that point of view, all injuries are permanent.
Well, yes. But the difference comes in the nature of the deficit you can't get back.
By not sleeping, you magically turn some fraction of the previous day's learning into wasted time, just throw it away.
The appropriate analogue in the twisted ankle case would be if the twist magically "un-walked" the 4 miles you walked the day before, and that increment of weight loss and cardio health were lost to you. This is of course not the case.. :)
From that point of view, all injuries are permanent.