The main ingredient in NyQuil that causes sleepiness is doxylamine succinate, which is an antihistamine found in a variety of over-the-counter cold and flu medications. Dextromethorphan, a cough suppressant, is another active ingredient in NyQuil that can cause drowsiness. It is important to note that NyQuil should not be used as a long-term sleep aid, and it is best to avoid combining it with other sleep aids or alcohol, as this can increase the risk of potentially risky sedative effects.