Why do we sleep? from birth, we (spend) a third of our lives asleep but scientists srill (not/know) exacly why.
why (humans/have) problems sleeping? in modern society, many people (not/get) the recommended seven or eight hours a night. we (work) long hours and we rarely (go) to bed at sunset.
why (we/sleep) differently? it (depend) on the time of year and also our age. teenagers always (need) more sleep than adults. lots of elderly people (not/sleep) longer than four or five hours, but they often (take) naps during the day.