light sleep vs forty winks

light sleep and forty winks both are nouns.

Word	Noun	Adjective	Verb	Adverb
light sleep	Yes	No	No	No
forty winks	Yes	No	No	No

As nouns, light sleep and forty winks are synonyms defined as:

light sleep and forty winks: sleeping for a short period of time (usually not in bed)

Other synonyms of light sleep include catnap, nap, short sleep, snooze.

light sleep (noun)	forty winks (noun)
sleeping for a short period of time (usually not in bed)	sleeping for a short period of time (usually not in bed)