Seven hours sleep is the magic number

2^nd August 2010 -- People who sleep for less than seven hours a day, including naps, are at a higher risk of cardiovascular disease. Sleeping fewer than five hours a day, including naps, more than doubles the risk of angina, heart attack, or stroke, according to a study conducted by researchers at West Virginia University’s (WVU) faculty of medicine.

People most at risk were those over the age of 60, who slept for five hours or fewer per night. Their risk of developing cardiovascular disease was more than three times that of those who slept for seven hours.

The study

The study, which was published in the journal Sleep, found that sleeping for more than seven hours also increased the risk of cardiovascular disease. Those in the study who slept for nine hours or longer were one-and-a half times more likely to develop cardiovascular disease than those who slept for seven hours.

The researchers, led by Anoop Shankar, associate professor at WVU’s Department of Community Medicine, analysed data from over 30,000 adults. The authors of the study were unable to determine the causal relationship between how long someone sleeps and the risk of cardiovascular disease. However, they noted that the duration of sleep affects endocrine and metabolic functions and lack of sleep can lead to high blood pressure, impaired glucose tolerance and reduced insulin sensitivity. These can all lead to hardening of the arteries.

A lie-in does you good

A separate  study, also published in Sleep, showed that an occasional lie-in can help those who routinely don’t get enough sleep. This study of 142 adults whose sleep was severely restricted for five days - as it can be for many people in the working week - showed that their reaction times were slower and they had more trouble focussing.  

However, having an extra hour or two of sleep in the morning after a period where sleep was restricted to four hours a night, resulted in a major improvement in these symptoms of sleep deprivation. The improvements were seen after just one night of recovery sleep.

"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, “ says David Dinges, head of the sleep and chronobiology unit at the University of Pennsylvania’s School of Medicine, who led this study. Have that weekend lie-in. It's not lazy, it's good for you. But be warned - for severe sleep deprivation, an extra 10 hours sleep may not be enough, according to the study.

Performance and alertness deteriorated profoundly when the five nights of restricted sleep were followed by a night of either no sleep or only two hours of time in bed, the study also showed. So all-night partying after a week of little sleep is not such a good idea.