“The Rebbe Rashab once said that the reason for not studying Torah on Nittel nacht is that ”one should not elicit an increase in life-force.” This restriction on study applies only until midnight.” (source)

"There are additonal customs associated with Nittel Nacht, such as eating garlic to ward off the demons (particularly you know whose), praying Aleinu out loud (since that is the prayer against idolatry), and not going to sleep all night." (source)