The question of how many hours of sleep are necessary to stay healthy is one that has for decades left researchers and sleepers tossing and turning.

But some scientists believe they have arrived at the magic number.

Daniel Gartenberg, a sleep scientist and assistant adjunct professor in biobehavioural health at Penn State University, told CTV News Channel that the optimal amount of sleep is 8.5 hours per night.

“To actually get those eight hours, you actually need to spend a little more time in bed than that,” Gartenberg said Tuesday. “A healthy sleeper will only spend around 90 per cent of the time in bed sleeping, so to get the eight hours that most people need, you need to spend 8.5 hours in bed.”

The American Academy of Sleep Medicine guidelines set the number at seven hours.

Gartenberg pointed to 2013 research from Gallup, a pollster, which shows that Americans sleep for an average of seven hours —one hour less than they did in the 1940s. According to data from Statistics Canada, Canadians aged 18 to 64 slept an average of 7.12 hours per night from 2007 to 2013.

While some people known as “short sleepers” can function on very little sleep, Gartenberg said that very few fit into that group.

Gartenberg said that the optimal number of hours of sleep needed can vary by individual and outlined a method that he and his colleagues use to determine the ideal number for each person.

“Before you go on vacation, fall into a regular pattern of sleep. Then when you go on vacation, you’re free of the stresses that force you to wake up earlier. Go to bed at the same time every night, and what will happen is that you’ll fall into this natural pattern, and that’s probably the amount that you individually will need,” he explained.

Studies show that sleep that is of short duration or poor quality is associated with a plethora of harmful health conditions, such as depression, diabetes, cardiovascular disease and obesity.

“There is some ability to catch up on sleep,” Gartenberg said. “But when we sleep deprive ourselves, we’re really hurting our bodies and there is nothing we can do to fully regain the damage that is done.”