What ingredient in nyquil makes you sleep?

The ingredient is Doxylamine succinate, which is an antihistamine. Personally, I prefer staying away from heavy chemical syrups for sleep; a light, timely dinner and avoiding caffeine after 4 PM works much better naturally.

I completely understand how frustrating sleepless nights can be, I used to worry about it constantly. Doxylamine in it makes you drowsy, but please don't use it regularly for sleep as it can become habit-forming. Talk to a doc if insomnia persists.

Doxylamine succinate is the one causing that heavy hangover sleep. Honestly, a good 45-minute workout or a run in the evening will tire you out naturally, giving you way better quality deep sleep without needing any cold syrups.

It is Doxylamine succinate, which is an sedating antihistamine used to relieve allergy symptoms. Since drowsiness is a major side effect, it makes you sleep. Please avoid driving or operating machinery after taking it.

Doxylamine succinate is what knocks you out, boss. But don't make it a habit to drink cough syrup just to fall asleep. Fix your sleeping schedule, keep your phone away, and use it only when you actually have a bad cold.