There's bad news for people who like to spend time under the covers on Sunday morning - a lie-in at the weekend makes you sleepier during the week, according to new research. A study led by Dr Gregory Carter, a specialist in sleep medicine at the University of Texas Southwestern Medical Centre, revealed that far from catching up on sleep, liers-in are in fact disrupting their circadian clock - the internal timer that regulates all kinds of human behaviours, including hormones and sleeping times. By sleeping in on a weekend, the researchers found that people may be delaying their circadian clock by up to two hours, making it harder to get to sleep on Sunday evening and to get up on Monday morning. Going to sleep earlier on Saturday is a more effective way of balancing your sleep debt, said Dr Carter. "Too many of us, however, stay up later on Friday and Saturday nights and choose to sleep in on Saturday and Sunday mornings," he added. "This pattern - combined with sleep-defeating actions that may include alcohol consumption and late-night checking of e-mails just prior to bedtime - makes for a painful Monday wake-up call."