Hi Cherry, I was curious about that too, so one night about a year ago I walked out onto the Ellipse adjacent to the White House and asked one of the policemen there that very question.
At that time he told me that they turned the White House lights (exterior) off at about 11:00 p.m.
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The light on the flag should never be off. So is it off? If so please explain? Some upgrades and repairs like at Blair House maybe? haha