nightcap; drink before bed
A drink, typically alcoholic, taken to help one fall asleep. Often implies a small amount of sake or other alcohol consumed shortly before going to bed.
I had a little whiskey as a nightcap.
寝酒の習慣はあまり良くないらしい。
They say the habit of having a drink before bed isn't very good.
Compound of 寝 (ne, 'sleep') and 酒 (sake, 'alcohol'). The exact historical derivation is uncertain, but the word transparently means 'sleep sake'.