Many people, such as students and office workers, sleep little on weekdays but sleep much longer on weekends. People believe extra weekend sleep can fully repair the damage caused by irregular daily sleep and broken body clocks, which is called social jetlag. This paper uses basic sleep science and simple experiment results to check if this idea is true. Human sleep has two control systems: sleep pressure and the 24-hour body clock. Weekend sleep can ease tiredness from short weekday sleep for a short time by making deep sleep longer. However, if people wake up over two hours later on weekends, their body clocks will become more disordered. Three natural biological limits stop weekend sleep from fixing long-term clock damage. In short, sleeping in on weekends cannot fully heal health problems from messy weekday sleep. Keeping a steady sleep time every day is the better choice. This paper may give easy sleep tips for students and adults and help protect their body clocks.