NyQuil makes you sleepy primarily because it contains an antihistamine called doxylamine succinate. This ingredient causes drowsiness by blocking histamine receptors in the brain, which helps reduce allergy symptoms but also promotes sleep by affecting neurotransmitters involved in wakefulness. This sedative effect is intentional as NyQuil is formulated for nighttime use to help people sleep despite cold and flu symptoms. Additionally, NyQuil contains dextromethorphan, a cough suppressant that can also cause drowsiness as a side effect, and acetaminophen for pain relief, which does not cause sleepiness. The drowsiness typically starts about 30 minutes after taking NyQuil and lasts for 6 to 8 hours, but it can be longer depending on metabolism, age, and liver function.

Key Points:

• Doxylamine succinate blocks histamine receptors, causing sedation.
• Dextromethorphan also contributes to drowsiness.
• NyQuil is designed for nighttime use to promote restful sleep during illness.
• Sleepiness duration varies among individuals and can last several hours.
• The sleepiness from NyQuil is not dangerous if used as directed but can impair alertness.

This explains why NyQuil is effective in making users sleepy and helping them rest while dealing with cold or flu symptoms.