To expand a bit more. When NR is turned on for long exposures, the camera automatically takes a second long exposure with the shutter closed (black frame) in order to identify camera-generated noise and hot pixels. The camera then used the 2nd photo to subtract camera-generated noise and hot pixels from the 1st photo.
As Tom stated, it takes a while for all this to happen.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"