Why do ....we sleep....(we / sleep)? From birth, we 2............. (spend) a third of our lives asleep but scientists still 3.............(not/know) exactly why.
Why 4...........(humans / have) problems sleeping? In modern society, many people 5.......(not/get) the recommended seven or eight hours a night. We 6.......(work) long hours and we rarely 7........(go) to bed at sunset.
Why 8........(we/ sleep) differently? It 9.......(depend) on the time of year and also our age. Teenagers always 10.............(need) more sleep than adults. Lots of elderly people 11........(not/ sleep) longer than four or five hours, but they often 12.........(take) naps during the day.
