Anybody else find it easy to forget...

Maybe somebody who's well versed in the facts of Hiroshima can explain this, we've seen in all the nuclear holocaust movies, bombs hit, sun disappears, and in this movie when the bomb hits it's like night even though it's only I guess about 8 o' clock in the morning. Granted, we don't really know how much time passes from the time Gen gets up to when they discover the people are dying from drinking the water. But every time I watch it I forget that this isn't the middle of the night, it's the morning and probably the afternoon that everything is pitch black, but the next day the sun's out again, why so quickly?