The main difference between Nyquil and Zzzquil is that Nyquil can be used for relieving cold, cough, fever, and sleeplessness. Whereas, Zzzquil can be used only for having a good night's sleep and does not provide relief from other issues. They both have a common ingredient for sedating called antihistamines.
Though NyQuil contains components that may lead to better sleep as a side effect, relying on the medication for sleep is not advised. Like any drug that can assist with either falling asleep or maintaining sleep throughout the night, using NyQuil regularly may lead to several problems.