Nyquil is a medication used to treat cold and flu symptoms, and it can also cause drowsiness. The main ingredient in Nyquil that causes sleepiness is doxylamine succinate, an antihistamine that blocks histamine from attaching to receptors in the brain, which results in drowsiness. Dextromethorphan, a cough suppressant, is another active ingredient in Nyquil that can cause drowsiness. Acetaminophen, a pain reliever and fever reducer, is also included in Nyquil, but it is not known to cause drowsiness. The effects of Nyquil usually start within 30 minutes of taking the medication, and they can last for about 6 hours. It is important to avoid driving or operating machinery after taking Nyquil, and it should not be combined with other sleep aids or alcohol, as this can increase the risk of sedative effects. While Nyquil can help with sleep when taken for cold and flu symptoms, it is not recommended for use as a general sleep aid. If you are having difficulty sleeping, it is best to talk to your doctor or healthcare provider for advice on how to improve your sleep.