Perhaps it was less physical exhaustion than emotion, which can make huge drains upon the system.

Now he was in the keeping of nature which was restoring all his powers of both mind and body, and keeping him there until he should again have all his strength and all the keenness of his faculties, needful for the great work that lay before him.
It was halfway toward noon when he awakened, remembered dimly in the first instant, and then comprehending everything in the second.

He unrolled the blankets, slipped out of his lair and knew by the height of the sun that he had slept far beyond the time appointed for himself.

But he did not worry over it.

Barring a little stiffness, which he removed by flexing and tensing his muscles, he felt very strong and capable.