There is some unwelcome news for students preparing for exams and officers putting in long hours-----you don't need the break as much as you may think that makes you feel less tired．

Scientists have long assumed that willpower （意志力） is a limited resource，which is why you feel the need to have a rest，have a snack and come back to a task when you're feeling better．They argued that the only way to restore willpower was by rest，food or entertainment．

But psychologists have challenged this theory，saying weak willpower is all in your head．They found that people's beliefs in willpower determine how long and how well they'll be able to work on a tough mental exercise．"If you think of willpower as something that's limited，you're more likely to be tired when you perform a difficult task，"said Prof．Veronika Job．"'But if you think of willpower as something that is not easily used up，you can go on and on．"

The researchers designed four experiments to test students'beliefs in willpower．After a tiring task，those，who believed or were led to believe that willpower is a limited resource，performed worse on standard concentration tests than those who thought of willpower as something they had more control over．They also found that leading up to final exam week，students who believed the limited resource theory ate junk food 24percent more often than those who believed they had more control in resisting temptation （诱惑）．

Mr．Job said．"The theory that willpower is a limited resource is interesting，but it has had unintended consequences．Students who may already have trouble studying are being told that their power of concentration is limited，and they need to take frequent breaks．But a belief in willpower as a non-limited resource makes people stronger in their ability to work through challenges．'"

The findings could help people who are battling temptation．Willpower isn't driven by a biologically based process as