My guess is that then-V.P. Harry Truman imagined he'd be president sooner than later.  President Roosevelt's health was not a public issue, but everybody in Washington knew he was a sick man.  It was a matter of time when the nation was fighting a war in Europe and the Pacific.

On April 12, V.P. Truman became President Truman.  Two weeks later, President Truman learned about what it really to be president.  On this day in 1945, he was briefed on whatever was going on in New Mexico:

America's secret development of the atomic bomb began in 1939 [sic -- it was 1941] with then-President Franklin Roosevelt's support. The project was so secret that FDR did not even inform his fourth-term vice president, Truman, that it existed. (In fact, when Truman's 1943 senatorial investigations into war-production expenditures led him to ask questions about a suspicious plant in Minneapolis, which was secretly connected with the Manhattan Project, Truman received a stern phone call from FDR's secretary of war, Harry Stimson, warning him not to inquire further.)

When President Roosevelt died on April 12, 1945, Truman was immediately sworn in and, soon after, was informed by Stimson of a new and terrible weapon being developed by physicists in New Mexico. 

In his diary that night, Truman noted that he had been informed that the U.S. was perfecting an explosive great enough to destroy the whole world.

On April 24, Stimson and the army general in charge of the project, Leslie Groves, brought Truman a file full of reports and details on the Manhattan Project. They told Truman that although the U.S. was the only country with the resources to develop the bomb–eliminating fears that Germany was close to developing the weapon–the Russians could possibly have atomic weapons within four years. They discussed if, and with which allies, they should share the information and how the new weapon would affect U.S. foreign-policy decisions. 

Truman authorized the continuation of the project and agreed to form an interim committee that would advise the president on using the weapon.

Do you wonder how he slept that night?

Looking back, I have a couple of thoughts.

Would a modern U.S. senator have kept the whole thing quiet as then-senator Truman did in 1943?  What a story to leak to a friendly reporter.  It would have hurt the war effort, and Truman was willing to give his commander in chief the benefit of the doubt.  It was a kind of honor and seriousness missing today.

President Truman did make the decision to use the bomb.  He understood that it had to be done to the end the war sooner rather than later.  Nevertheless, it must have been a heck of a meeting when he learned that the decision would be his to make.

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