Even if you've spent the weekend relaxing in bed, Mondays always seem to be a bit challenging. What's most surprising is when you've slept in over the weekend with the specific goal to recharge your batteries after a long week and to prep for the week ahead. Despite all that extra rest, Monday morning's alarm is still that much more piercing to your ears.

But sleeping in on the weekend is precisely the problem, researchers say. Sleeping later on the weekends than you usually do throughout the week confuses your body, throwing your circadian rhythm out of whack. Susanna Jernelöv, a psychologist and sleep researcher at Sweden's Karolinska Institute, says it's essentially like giving yourself mild jet lag.

"If you sleep a little bit too little all the time, and just catch up on the weekends, you are messing with your circadian rhythms," Jernelöv told Swedish newspaper The Local. "You should stay on a regular schedule, but that doesn't really work with most people's lives."

Switching up your sleep schedule isn't just making you groggy come Monday morning. A study published in the Journal of Clinical Endocrinology and Metabolism last year found it could also have negative effects on your health.

The researchers measured cholesterol levels and triglycerides of those who participated in the study. Those who had more extreme changes in their weekday and weekend sleep schedules had less good cholesterol and higher amounts of fatty substances in their blood. They also gained more weight. Over time, the researchers concluded, this could lead to an increased risk of diabetes and heart disease.

The solution? Try to wake up at the same time every single day. And of course, try to get a decent amount of sleep each night. So that leaves you with two options. Either drag yourself out of bed earlier on the weekends. Or, convince your boss to let you sleep late throughout the week. If it's a tough sell, tell her science said so.