It's not just restlessness--it makes sense to move on, of course, on the chance that their attackers might come back with reinforcements. But no one consulted the horses about their extra midnight shift, and they're more inclined to amble than to trot; and indeed fatigue hits William hard and suddenly about an hour after they're back on the road.

...Harry may not be terribly surprised to notice William nodding off in the saddle.