What is this 12 o’clock joke on Sikhs?

There is no reference to this in the Guru Granth Sahib Bani.
As best as I know, during the time when Afghani invaders came to India looted the wealth and took the Indian ladies back with them, Sikhs were furious and would attack the Afghani forces on their way back and free the captured ladies. The attack by the Sikhs was always planned to be at midnight as it was easy for everyone to know that midnight by looking at the stars. Because, Sikhs were few in numbers they had to fight like crazy, totally focused on their goal without paying attention to even the dangers to their own life. Yes, they went crazy / mad focussed on their objective risking their own life against heavy odds. Their goals was to take back some of the loot, free the ladies, and hurriedly get out of the reach of the Afghan army.
It bacame popular among locals that at 12 O’Clock midnight Sikhs are oming out of the hidings like crazy and they are mad to risk their life aginst such heavy odds. No one could stop them from invading the Afghan army even in the face of certain death.
Later on, the people only remember that Sikhs go mad / crazy at 12 O’ Clock and forgot all the rest.
That is how a brave practice of Sikhs is now turned into a joke by the ignorant.
Hope this helps.