How many hours of shuteye is best? Here's what the latest science says about sleep

With the release of two studies that try to untangle the relationship between sleep and brain health, specialists are offering some reassurance for anyone who's feeling anxious about optimizing their shuteye.

New research published Monday suggests that getting less than the recommended seven or eight hours of sleep each night might not be as harmful as expected for some people's brains.

That follows a study printed last week that suggestsdaytime napping could have some positive effects.

But sleep science is still a relatively new field, with much more research still to be done on how sleep patterns intersect with human health, according to a range of Canadian experts who weren't involved in the two new studies.

The evidence so far suggests that sleep needs can be very different for different people, said Dr. Elliott Lee, a sleep specialist at The Royal, Ottawa's mental health centre.

"If you find your sleep pattern to be good for you, then I think that's all right," he told CBC News.

That's a view shared by Dr. Ram Randhawa, a psychiatrist with the University of British Columbia'ssleep disorders program.

"My best advice is don't worry. Step back from all of this news; stop being so fixated on sleep performance," he told CBC News.

"Don't judge your sleep based on some measure of what the perfect or ideal sleep should be. Everyone's different."

'No one-size-fits-all rule' for napping

Chronically poor sleep can have serious consequences for physical and mental health, according to Dr. Michael Mak, a sleep medicine specialist at the Centre for Addiction and Mental Health in Toronto. It's been associated with cardiovascular problems, high blood pressure, high cholesterol, obesity, depression and anxiety.

What exactly constitutes good sleep habitsis still up for debate. Growing interest in the benefits of a healthysnooze has fed a booming industry, offering everything from wearable tech that tracks your hours of sleep or soothes you with soft vibrations, to weighted blankets and bed cooling systems.

Napping is something that sleep doctors traditionally advised against for their patients, the specialists consulted for this story said. The thinking was that sleeping during the day contributes to problemspatientshad sleeping at night.

But, Mak said, "there is no one-size-fits-all rule as it pertains to napping."

A June 19 paper in the journal Sleep Health suggests a modest link between a larger brain volume and people who reported napping regularly. The volume of a person's brain tends to decline with age, and faster shrinkage has been associated with memory problems and dementia.

The international team behind the study used information drawn from the U.K. Biobank, a database containing a wide range of biomedical information from 500,000 volunteers.

Interestingly, the scientists did not find any relationship between habitual napping and reaction time, visual memory or the volume of the hippocampus a part of the brain that is essential to memory.

But the larger overall brain volume in habitual nappers "could suggest that napping regularly provides some protection against neurodegeneration by compensating for poor sleep," the researchers wrote.

Dr. Brian Murray, the head of neurology at Sunnybrook Health Sciences in Toronto, pointed out that previous studies have also shown napping can help with cognition in people learning new things.

"People can practise learning a new skill all day, and then they take a nap and they get way better. So there is something about sleep that helps consolidate and organize the brain," he said.

The new research, however, raises some significant questions. It didn't look at how long the habitual nappers are sleeping or what time of day they're doing it both factors that doctors say could have an effect on the results.

Mak described the results as interesting and worthy of further research, but he cautions against napping for more than 20 minutes at a time. Any longer can lead to a state of disorientation and mental slowness that scientists call sleep inertia.

"If you feel groggy and sleepy during the daytime, you might choose to take a short nap and then if you respond well to it, that might be something you choose to do on occasion," Mak said.

Randhawa noted that napping is common in many cultures around the world, but anyone who doesn't enjoy a midday slumber shouldn't feel compelled to change their routines.

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New sleep study examines how brains function on less sleep

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A new sleep study in the journal Neuroscience looked at how brain function is affected by sleeping for shorter times at night and suggested there was slightly lower cognitive function across the board for people who sleep less than six hours each night.

"The people who are napping are napping not only because they want to, because it's a preference or a choice, but also because they find it relaxing, they find it enjoyable, they find it restorative," Randhawa said.

"If you try to force yourself to nap, it just leads to you lying in bed frustrated."

He also cautions against reading too much into studies like this, saying the reasons for any correlation between napping and brain health are far from clear.

Study raises questions about length of nighttime sleeps

The second study, published Monday in the Journal of Neuroscience by a team of European researchers, looked at how brain function is affected by sleeping for shorter times at night.

It draws on data from a number of biomedical databases, including the U.K. Biobank, comparing self-reported sleep with brain MRIs and cognitive tests.

The researchers found that people who sleep less than six hours a night and reported no daytime drowsiness or sleep disturbances had larger regional brain volumes compared to people who sleep seven to eight hours, or those who sleep less but also have sleep issues.

At the same time, however, testing showed slightly lower cognitive function across the board for people who sleep less than six hours each night.

"This indicates that sleep need is individual, and that sleep duration per se is very weakly if at all related [to] brain health, while daytime sleepiness and sleep problems may show somewhat stronger associations," the authors wrote.

Lee saidthese results shouldn't be applied too widely, pointing out that the more encouraging findings for short sleepers involved just a small fraction of the samples included in the study 740 out of more than 47,000 people.

For the vast majority of people, seven to nine hours of sleep a night is still ideal, he said.

"Anything below six hours pretty consistently, except for a small minority of people, can demonstrate problems in cognitive function [and]reaction time, compared to their baseline," Lee said.

Murray said the amount of sleep someone needs to function well can vary widely from person to person. In some cases, genes play a major role.

"For example, there are mutations that lead to very short sleep, so people can get by on quite a few less hours maybe four hours," he said.

"Others are long sleepers, so there are extreme variations."

Ultimately, though, Murray said research like this lays bare some challenges with studying sleep. Both recent studies relied on self-reported data about sleep habits, rather than objective measures.

"It is actually impossible for people to introspect, to think about what their sleep is like, because they are unconscious for that state," he said.

"Sometimes people's perceptions of what happened are very different."