In the 17th century, during the mughal occupation of India, non-muslims were humiliated and oppressed apart from being looted. Mughals treated the hindu women as their own property and were forcing the hindus to accept islam, resorting to manslaughter if people refused to accept.
The ninth Guru Sri Guru Teg Bhadarji came forward, under the request of few kashmiri pandits, to fight against this large scale oppression. Guru Teg Bhadarji told the mughal emperor, that if he succeed in converting the Guru to islam, all hindus would accept the same, but that if he failed, the emperor should stop these large scale conversions and cruelty. The mughal emperor happily agreed. Despite the long and several torture sessions to both the Guru and his fellow members, the emperor failed in his attempt to convert them to islam. The ninth Guru and his four fellow members were thus assasinated in Chandni Chowk. Guru Teg Bhadarji had sacrificed his life for the sake of Hinduism. Today the ninth Guru is remembered has `Hind Ki Chaddar'.
Torn by the death of the ninth guru and enraged by the inaction of the crowd, the 10th Guru Sri Guru Gobind Singhji (Son of the ninth Guru) made a resolution that he would make such a human, who would not be able to hide himself nor his action, one who would be easily located easily among the masses - THE SIKH.
At the start, sikhs were few in number and yet were fighting against mughal imperialism. In 1739 Nadir Shah raided Delhi, looting Hindustan of several treasures and taking nearly 2200 Indian women with him. The news eventually fell into the ears of Sardar Jassa Singh, then the commander of the sikh army. Sardar Jassa Singh decided to attack Nadir Shah's khafila on the same midnight, he did so and rescued the Indian/hindu women, who were subsequently sent to the safety of their homes. These midnight attacks on the mughals by the sikhs soon became the norm of their revoultion, during the several looting sessions of the raiders from the Arab world.
A string of such incidents put terror into the mughal hearts and hope back into the hindu masses. Tall stories of the sikh power were narrated and their help was sought, during the many skirmishes of the time. This continued and became a fame, that at midnight nearly at 12 O'clock, the sikh received extra power to save religion and nation. It was as though that the sikhs were possessed at 12 midnight. This soon, thanks to the handiwork of comedians and the sikh enemies alike, translated into the sikh loosing his senses at 12 0' clock.
The phrase `Saardarji barah baj gaye' soon became universal to both people who didn't like them as well as their admirers.