tablet vs sleeping pill

tablet and sleeping pill both are nouns.

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

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

tablet: 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 tablet include bolus, capsule, dragee, sleeping capsule, sleeping draught, sleeping tablet.

tablet (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)
a small flat compressed cake of some substance
a slab of stone or wood suitable for bearing an inscription
a portable computer with a touchscreen
a number of sheets of paper fastened together along one edge