Dr. Nate Watson, a scientific advisor for SleepScore Labs, explained the actions of the two substances: "Melatonin is typically taken to address circadian rhythm sleep disorders or as an insomnia therapy. Alcohol, although causing sleepiness initially, can cause sleep disruption/awakenings once it wears off throughout the night resulting in further insomnia troubles" (via Bustle). 

Medical News Today explains that while the supplement and alcoholic beverage combo may induce excessive sleepiness initially, the complications from the two can make for an unpleasant night — sleeplessness (after the initial drowsiness), irritability, and anxiety. And even more concerning, there is the potential for difficulty in breathing, dizziness, and passing out — which means a person taking melatonin and drinking alcohol may have trouble walking and should not attempt driving. Bottom line: "I recommend not drinking alcohol and taking melatonin," Watson said. "They both may cause drowsiness and when taken together, other symptoms ... can be problematic."

If sleep trouble is the reason for combining these substances, it's important to ask your doctor how to better improve your night's rest.