What's a word for a 'thing that makes one sleepy'?

Solution 1:

Most contemporary words have already been given, though 'hypnotic' is still used in pharmacology (as when distinguishing sleep-producing effects from sedation); the rest of these words are obscure.

• hypnotic : a drug which produces sleep (i.e. sedative)
• dwale : a stupefying or soporific drink (often specifically Belladonna) [TFD]
• sopient : a soporific agent (medical) [OED]
• somnifacient a drug or other agent that induces sleep.
• somniferic : a soporific [seemingly conflated as another form of the adj somniferous, but cited as an obsolete noun by OED]
• somnificator : a person who induces sleep [OED, rare]
• somnivolency : a thing intended as a soporific (pl. somnivolencies) [OED, rare; a somnivolent is also one who desires] to sleep
• somnoriferous/somnorific : adj. soporific, as said of an agent [OED, obs]
• soporative : obs form of soporific [OED]

And a few more adjectives, since they can be easily made to refer to such a thing:

somniculous, somnific, somnifying, soporous,

Solution 2:

Soporific, or soporiferous:

• Inducing or tending to induce sleep.
• Drowsy.

(AHD)

• The professor’s boring speech was soporific and had everyone in the audience yawning.

• Because of the medicine’s soporific properties, the doctor told me to only take it at bedtime.

Solution 3:

'Sedative' is a word for a thing (substance, pill, medicine, drug) that makes one sleepy, because it induces (or tends to induce) sleep.