I have the dcs-5222l, says on the box "view up to 15 ft in darkness with IR LEDs".
Now, my camera is behind a window, the IR LED's reflect off the window. I have my IR LEDs covered up and around 4.30PM were it's a bit dark with street lights on I can detect motion in night mode.
Please note it's dark outside but not pitch black, I'm only really posting as I didn't think of the LED light's reflecting off the window before I brought my IP camera, which makes it pointless to have night mode on.
When I first got the camera and tested it out, night mode on and in a large room it detected movement just fine.