Text 4 When you go to bed,is it because you"re tured or because you need to get up at a certain time and want to make sure you get enough sleep?Everyone has a chronotype,which is the sleep cycle that their body would naturally prefer,if left to its own devices.But society forces its own chronotype on people,too.Maybe your prefer to sleep from 2 a.m.t0 10 a.m.But if you have a typical 9-to-5 workday,to get eight hours,you"d probably need to sleep from something like 11 p.m.t0 7 a,m.Individuals"sleep is surely shaped by their jobs,their families,and their habits.But a new study shows that society can shape sleep broadly,on a population level,as well.In the paper,published in Science Advances,Olivia Walch,Amy Cochran,and Daniel Forger of the University of Michigan look at data gathered from a smartphone app to see how sleep cycles vary in different countries and among different demographics.What country people lived in didn"t have any noticeable effect on when they woke up,but it was linked to what time they went to bed.And what time they went to bed was linked to how much sleep they got.So in the countries that got less sleep on average,like Japan and Brazil,it was because they were going to bed later,not because they were waking up earlier than people in countries that got more sleep on average,like the Netherlands and Belgium,where people have earlier bedtimes.But it"s notable in pointing out bedtime as a problem area.A lot of the research and discussion about the tension between sleep and society has focused on the morning-how the 9-to-5 workday isn"t suitable for everyone"s internal clocks,how starting the school day later can help teens,who have notoriously late chronotypes,how exposing yourself to light in the moming can help keep your intemal clock wound in a way that promotes good sleep.Bedtime,meanwhile,seems like it should be more of a choice.You wake up to go somewhere,or to start fulfilling responsibilities,and you go to bed when you decide to,based on a complex calculus of how tired you are,how much sleep you want to get,and whether the chapter you were just reading in your book ended on a cliffhanger.But when people are deciding to go to bed differs significantly across societies,it seems there"s something cultural at play here,too. The factors affecting people"s sleep time do not include____