[3][4] The most prominent custom commonly observed on Nittel Nacht is to abstain from Torah study, although historically some read the Toledot Yeshu instead.
[1][6] Some Jewish mystics believed apostates were conceived on the day and as a result forbade married couples from sexual relations on Nittel Nacht.
"[9][4] The first allusion to the practice of staying up late playing games appears in a Jewish communal ordinance from 1708 and was later mentioned in the work of Moses Sofer.
[13] Medieval apostates such as Johann Pfefferkorn, Julius Conrad Otto, Johann Adrian, and Samuel Friedrich Brenz wrote that the common belief among Jews at the time was that on Christmas Eve, Jesus would wander all the toilets of the world as a punishment for spreading false teachings.
They wrote that Jews feared that if Jesus heard them reading the Torah, he would get a respite from his suffering, so they refrained from it.