We tend to think of Monday as the worst day of the week because of the substantial change in attitude that Monday mornings bring. Till Roennenberg, a professor of chronobiology at Ludwig-Maximilian University in Munich, developed the phrase "social jet lag" in 2006. It alludes to the way our bodies struggle to adjust to the time zone we were in over the weekend while our obligations keep us firmly grounded in the workweek.
This occurs when we disrupt our habits and throw off our internal clocks during the weekend, frequently by trying to make up for our accumulated "sleep debts" from the previous week by oversleeping. This causes us to feel exhausted on Monday mornings. But weekend oversleeping isn't the only offender here. The dissonance that results from giving in to the sweet freedom of ungodly late nights and leisurely mornings while we savor the absence of alarms and work-related stress, abandoning our weekday sleep schedule in the process, and transforming into nocturnal creatures, can all be difficult to overcome. Drinking and partying over the weekend may also leave us with a hangover to deal with on Monday mornings in addition to a social drain.
"Staying up late at the conclusion of a difficult work week and sleeping in on the weekend may both disrupt your circadian rhythm, your body's natural schedule, much like moving from New York to Los Angeles might occasionally do. You're effectively putting your body into a foreign time zone by staying up late on Friday and Saturday nights then sleeping in both days the next day, says Raj Dasgupta, a sleep expert at the USC Keck School of Medicine.