Waking after a bad night often comes with a comforting promise: more sleep at the weekend will make everything right. Research suggests extra rest can help, but sleeping in or going to bed early can also disrupt the systems that regulate sleep.

Nearly half of almost 8,000 US adults surveyed said they regularly slept in on weekends. Teenagers appear to have the biggest weekend sleep extensions of any age group.

Adults are generally advised to get around seven to nine hours of sleep, while most adolescents are thought to need eight to 10 hours. When weekday demands make those targets difficult, extra weekend rest can be tempting.

The complication involves two processes that regulate sleep: the sleep homeostat and the body clock. Sleeping later reduces the pressure to sleep that builds while a person is awake, potentially making it harder to feel tired at the usual time that evening.

A changed schedule also shifts activities such as light exposure, which can affect the circadian rhythm. Someone who sleeps late on Sunday may fall asleep later that night, then wake at 06:00 on Monday without having had enough rest.

"It's like shifting time zones every single week," says Melynda Casement, sleep psychologist and director of the University of Oregon Sleep Lab. That recurring disruption can restart the cycle of weekday sleep loss.

Even so, controlled experiments indicate that recovery sleep can blunt or sometimes reverse certain consequences of a particularly difficult night. In one study, people restricted to two hours of sleep showed elevated counts of two types of white blood cells, potentially indicating a physiological stress response.

Participants allowed eight hours of sleep the following night did not fully return to their earlier white blood cell levels. Those permitted up to 10 hours had levels that had nearly returned to baseline.

Another experiment examined 11 men who normally slept eight hours nightly but were limited to four hours for six nights. Their glucose clearance and insulin responses became impaired, changes that can raise the risk of type 2 diabetes if sustained over the long term.

After six additional nights with a 12 hour sleep opportunity, those measures returned to baseline. Other experiments found that recovery sleep after staying awake all night could restore cognitive performance.

Still, extra weekend sleep does not erase every consequence when insufficient sleep continues for weeks or months. Vigilant attention, including the ability to remain focused and respond quickly, and spatial orientation appear particularly vulnerable.

Evidence from daily life presents a more complicated picture. A Swedish study followed more than 40,000 adults for 13 years and found that participants younger than 65 who regularly slept six to seven hours had the lowest mortality risk.

Those sleeping five hours or less had a 65% greater mortality risk, but adequate weekend sleep appeared to eliminate that additional statistical risk. Among adults older than 65, researchers found no association between short sleep and mortality risk, which they suggested reflected decreasing sleep needs with age.

A study of almost 2,800 Korean adults found that habitual short sleepers were about 73% more likely to have high blood pressure. Each additional hour of weekend recovery sleep was associated with about 17% less risk.

Other research linked weekend sleeping in among short weekday sleepers with lower risks of dementia and metabolic syndrome. However, a large UK based study found no link between weekend recovery sleep and mortality or cardiovascular disease, while also finding no greater risk from sleeping in.

Till Roenneberg, professor emeritus of chronobiology at LMU Munich's Institute for Medical Psychology in Germany, rejects advice that everyone should use an alarm on weekends to preserve a rigid schedule. "Of course you have to catch up on sleep," he says. "Sleep is one of the most important functions in our lives."

People with insomnia are an exception because usual treatment recommends avoiding both earlier bedtimes and sleeping in, even after a difficult night. For most other people, growing evidence points to roughly one to two additional hours as a possible sweet spot.

That amount has been associated with the lowest risk of depression in US and Korean studies, lower inflammation among adults who habitually sleep too little, and reduced anxiety and depression risks in younger people. A modest extension may therefore help, but the evidence offers little support for sleeping until lunchtime.

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