noun
sleeping pill; sleeping tablet
A medication taken to help fall asleep.
昨夜はどうしても眠れなくて、睡眠剤を飲んだ。
Last night I just couldn't sleep, so I took a sleeping pill.
睡眠薬 is a more common synonym for sleeping pill; 睡眠剤 is slightly more formal or medical.
Compound of 睡眠 (suimin, 'sleep') and 剤 (zai, 'medicine, drug').