The physical relaxation after such hard marching and the sharp wind of the night made Harry shiver, despite his blanket.

St.Clair and Langdon shivered, too.

They did not know that part of it was that three-o'clock-in-the-morning feeling.
Harry, sensitive, keenly alive to impressions, was oppressed by a certain heavy and uncanny feeling.

They were going into battle in the morning--and with men whom he did not hate.

The attacks on the Star of the West and Sumter had been bombardments, distant affairs, where he did not see the face of his enemy, but here it would be another matter.
The real shock of battle would come, and the eyes of men seeking to kill would look into the eyes of others who also sought to kill.
He and St.Clair were not sleepy, as they had slept through most of the day, but Langdon was already nodding.