Why do we sleep (we/sleep)? From birth, we ___ (spend) a third of our lives asleep but scientists still ___ (not/know) exactly 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) different?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, bu they often ___ (take) naps during the day.