There's enough doxylamine succinate to make you uncomfortably tired before you feel high. DXM is overrated but RoboCough is the best bang for your buck without extra shit in it if you're angling for a night that feels like teenage regret.
Nyquil brand specifically made the non-addictive Zzzquil for people who just want that wibbly woobly sleepy feeling. I think it's just melatonin. If taken in excess, could probably fuck up your body's natural melatonin production therefore making sleeping problems worse.