pill vs sleeping pill

pill and sleeping pill both are nouns.

Word	Noun	Adjective	Verb	Adverb
pill	Yes	No	No	No
sleeping pill	Yes	No	No	No

As nouns, sleeping pill is a hyponym of pill; that is, sleeping pill is a word with a more specific, narrower meaning than pill:

pill: a dose of medicine in the form of a small pellet
sleeping pill: a soporific drug in the form of a pill (or tablet or capsule)

Other hyponyms of pill include bolus, capsule, dragee, sleeping capsule, sleeping draught, sleeping tablet.

pill (noun)	sleeping pill (noun)
a dose of medicine in the form of a small pellet	a soporific drug in the form of a pill (or tablet or capsule)
something that resembles a tablet of medicine in shape or size
a contraceptive in the form of a pill containing estrogen and progestin to inhibit ovulation and so prevent conception
something unpleasant or offensive that must be tolerated or endured
a unpleasant or tiresome person