资料：It’s almost considered sacrilegious today to leave work at the end of your workday or（for shame！）on a Friday and simply not check your work email again until you return the office during normal working hours。 The constant need to check email is the trade-off the modern workforce has made for the ability to work anytime，anywhere，hanks to smartphones and tablets that keep us always connected。 But three university researchers have found that it’s not just doing a bit of work after hours that cause burnout。The true culprit is actually the constant worrying about off-hour email。 A new study。“Exhausted But Unable to Disconnect，”by Lehigh University’s Liuba Belkin，Virginia Tech’s William Becker and Colorado State University"s Samantha Conroy shows that employees are growing exhausted by the expectation that they will always be available，never knowing what kind of work requests will be asked of them off hours。 Typically，companies don’t mean to stress employees out like that。Most companies don"t have formal policies that say people must answer work emails after-hours，（except，perhaps，in cases where an employee is on call during specific times）。 But policies and culture tend to be two different things。If supervisors routinely email employees after hours and expect a fast response （often because their supervisors are doing the same to them），then the message is clear: whenever the boss emails，the employee is expected to be available。 The solution is for bosses to tell employees that an after-hours email doesn’t necessarily require a response before the next work day，and to also set some times when after-hours emailing is considered acceptable and prohibited，such as no emails via the dinner hour，on weekends，or after 10 p.m.，the researchers say。 What is this passage mainly about？