How much sleep do you really need? Read our chart to find out
The guidelines on what constitutes a good night's sleep have been revised. As little as five hours and as much as 11 hours may be appropriate for adults depending on age, doctors now say.
Just how much sleep do we really need? While new recommendations released last month have not changed - seven to nine hours each night for adults is still advised - experts now acknowledge that what constitutes a good night's sleep varies among individuals.
So they've come up with a new range: six hours of sleep "may be appropriate" for adults aged 18 to 64, says the expert panel convened by the US National Sleep Foundation in a report which was published in the foundation's journal, Sleep Health.
These new guidelines may come as relief for Hongkongers, many of whom are sleep deprived. The most recent Health Department statistics from 2011 found that 35.5 per cent of adults slept fewer than seven hours a day on average. Only 6.4 per cent of the 2,000 adults polled got more than eight hours of sleep a night.
Another survey released in 2010 of 5,000 Chinese adults in Hong Kong, led by Dr Wong Wing-sze, an associate professor at the Hong Kong Institute of Education, found that the average sleep duration was just six hours, 27 minutes.
But while some people may operate perfectly fine on just six hours of sleep, the National Sleep Foundation expert panel emphasises that seven to nine hours is still ideal.
"Individuals with sleep durations far outside the normal range may be engaging in volitional sleep restriction or have serious health problems," says the report. "An individual who intentionally restricts sleep over a prolonged period may be compromising his or her health and well-being."
The National Sleep Foundation convened 18 experts from sleep, anatomy and physiology, as well as paediatrics, neurology, gerontology and gynaecology to reach a consensus from the broadest range of disciplines.
Following a comprehensive review of 320 published scientific studies on sleep and health, multiple rounds of consensus voting were carried out to arrive at the final recommendations.
The new guidelines (see infographic) revised sleep ranges for all six children-and-teen age groups. All groups except newborns had recommended sleep ranges widened by an hour, while newborns had their sleep range narrowed to 14 to 17 hours each day (previously it was 12 to 18).
Adults were also stratified from one large category into three: younger adults (18 to 25 years), adults (26 to 64 years) and older adults (over 65).
"The process [of coming up with the recommendations] was very rigorous," says neuroendocrinologist Lydia DonCarlos, a professor at Loyola University Chicago Stritch School of Medicine in the US, who was part of the expert panel.
"We still have a great deal to learn about the function of sleep," she says. "We know it's restorative and important for memory consolidation. But we don't know the details of what the function of sleep is, even though it is how we spend one-third of our lives."
There is, however, increasing evidence of the havoc that insufficient sleep wreaks. Multiple studies have reported connections between restricted sleep, weight gain and type-2 diabetes, says Dr Esra Tasali, an assistant professor of medicine at the University of Chicago.
Tasali is the senior author of a study published last month in Diabetologia that shows in healthy young men, just three days of insufficient sleep disrupted fat metabolism and put the men in a temporary pre-diabetic state.
"Curtailed sleep produced marked changes in the secretion of growth hormone and levels of [the stress hormone] noradrenaline, which can increase circulating fatty acids," says the study's lead author, Josiane Broussard, a post-doctoral research scientist at Cedars-Sinai Medical Centre's Diabetes and Obesity Research Institute in Los Angeles.
"The result was a significant loss of the benefits of insulin. This crucial hormone was less able to do its job. Insulin action in these healthy young men resembled what we typically see in early stages of diabetes."
Lack of sleep not only causes changes in metabolic hormones, but also brain activity, a new University of Pennsylvania study shows. The study sequestered 34 sleep-deprived subjects and 12 controls in a sleep lab for five days and four nights for round-the-clock monitoring. The subjects were free to eat whatever they wanted and as much as they liked during the tests.
During a night of total sleep deprivation, subjects consumed close to 1,000 calories. Despite this, the following day, they ate a similar amount of calories as they did the day following a good night's sleep.
In addition, they ate a greater percentage of calories from fat and a lower percentage of calories from carbohydrates during the day following total sleep deprivation.
Sleep deprived subjects displayed increased connectivity within the "salience network", located towards the front of the brain, and this correlated with eating more fat. Activity in the salience network is linked to both emotion and bodily sensations, such as the heart racing, stomach churning, pain, thirst, embarrassment and attempting mental challenges.
Other recent studies have found connections between insufficient sleep and damage to cells, especially in the liver, lung and small intestine; increased risk of developing ulcerative colitis, a form of inflammatory bowel disease; and likelihood of an extended absence from work due to sickness.
The panel notes that while sleep duration is important, sleep's restorative properties undoubtedly also depend on sleep quality, sleep architecture and the timing of sleep within the day.
You should decide on your individual sleep needs by assessing how you feel after different amounts of sleep, advises the National Sleep Foundation.
Some questions to ask yourself
Are you productive, healthy and happy on seven hours of sleep? Or does it take you nine hours of quality sleep to get you into high gear?
Do you have health issues such as being overweight? Are you at risk for any disease?
Are you experiencing sleep problems?
Do you depend on caffeine to get you through the day?
Do you feel sleepy while driving?