sleep vs period of time

sleep and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
sleep	Yes	No	Yes	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of sleep; that is, period of time is a word with a broader meaning than sleep:

Other hypernyms of sleep include period, time period.

sleep (noun)	period of time (noun)
a natural and periodic state of rest during which consciousness of the world is suspended	an amount of time
euphemisms for death (based on an analogy between lying in a bed and in a tomb)
a torpid state resembling deep sleep
a period of time spent sleeping

sleep (verb)	period of time (verb)
be asleep
be able to accommodate for sleeping