Other forms: soporifics; soporifically
Something that is soporific is sleep-inducing. Certain medicines, but also extreme coziness, can have a soporific effect.
In the 1680’s, soporific, which doubles as both adjective and noun, was formed from the French soporifique. That word, in turn, came from the Latin sopor “deep sleep.” Beloved Peter Rabbit author Beatrix Potter once noted that, “It is said that the effect of eating too much lettuce is 'soporific'."