soporific [sop-uh-rif-ik, soh-puh-]
1 causing or tending to cause sleep; tending to dull awareness or alertness
2 of, relating to, or marked by sleepiness or lethargy
something that causes sleep, as a medicine or drug.
To avoid muddying the texture, pianists rely on a clean, detached style, and as a result the music too often sounds subdued, fastidious, even soporific. (Alex Ross, The Rebel Harpsichordists, New Yorker)
This never made sense anyway, though for the gullible few, inflamed rhetoric has apparently served as an effective soporific. (Thomas Jewell, A Noble choice: Carol Roe selected as new Cleveland Heights mayor)
The first pages of most of these old papers are as soporific as a bed of poppies. (Nathaniel Hawthorne, Old News)
1680s, from French soporifique (17c), formed in French from Latin sopor (genitive soporis) 'deep sleep' (from PIE root swep- 'to sleep'). As a noun from 1722. Earlier as an adjective was soporiferous (1580s as 'characterized by excessive sleep,' c 1600 as 'soporific').
The term traces to the Latin noun sopor, which means 'deep sleep.' (That root is related to somnus, the Latin word for sleep and the name of the Roman god of sleep.) French speakers used sopor as the basis of soporifique, which was probably the model for the English soporific. (Merriam-Webster)