If you wake up on Monday mornings feeling less sharp than usual it is probably because you do not treat yourself to long enough lie-ins at the weekend.
Scientists have discovered that an extra dose of sleep is more than just a luxury - it provides an essential boost to brain power ahead of the working week.
Those who return to work on Monday morning refreshed from a long lie-in the day before perform better than those who spent less time in bed at the weekend, research has found.
A single lie-in is all that is required to replenish the brain and boost energy, alertness and attention span after a week of restricted sleep, the study showed.
But even 10 hours in bed may not be enough to restore those who regularly burn the candle at both ends – so they should lie in even longer.
The scientists also warned against staying up all night partying at the weekend, which they said would significantly impair a person's performance at work the following week.
Dr David Dinges, chief of the Division of Sleep and Chronobiology at the University of Pennsylvania, who led the study, said: "The additional hour or two of sleep in the morning after a period of chronic partial sleep loss has genuine benefits for continued recovery of behavioural alertness.
"The bottom line is that adequate recovery is important for coping with the effects of chronic sleep restriction on the brain."
The scientists conducted a sleep deprivation experiment on 159 healthy adults, with an average age of 30.
After two nights of 10 hours in bed, 142 participants were restricted to four hours in bed from 4am to 8am for five consecutive nights.
They were then allowed randomised doses of recovery sleep ranging from zero hours to 10 hours for per night. The other 17 participants spent 10 hours in bed on all nights.
Participants were asked to complete 30-minute computerised assessments every two hours while they were awake. Those whose sleep had been restricted were found to have a shortened attention span, impaired alertness and reduced reaction time. But their normal functions were restored after just one full night of sleep.
The study also showed that people who got no sleep at all after the period of sleep deprivation performed significantly worse than normal.
"This highlights the importance of avoiding all-night sleep deprivation following a period of restricted sleep," Dr Dinges said.
The findings contradict the received wisdom that eight hours’ sleep provide the best start to the day.
The study of more than 30,000 adults found that cardiovascular disease – which includes heart attacks, strokes and angina attacks – is twice as high among those sleeping less than five hours a day, compared with those getting seven hours.
Those who slept nine hours or more also had a markedly increased risk.