Mr. Obama calls himself a "night guy," and as president, he has come to consider the long, solitary hours after dark as essential as his time in the Oval Office. Almost every night that he is in the White House, Mr. Obama has dinner at 6:30 with his wife and daughters and then withdraws to the Treaty Room, his private office down the hall from his bedroom on the second floor of the White House residence. There, his closest aides say, he spends four or five hours largely by himself. He works on speeches. He reads the stack of briefing...