With so much to do heading into the busy holiday season, is anyone really getting enough sleep?

Most sleep experts advise that adults get seven to nine hours of sleep per night for good health and emotional well-being, although that recommendation changes as you get older. Studies warn that sleeping less than seven hours a day can increase the risk of obesity, high blood pressure, heart disease and other problems associated with sleep deprivation.

Research has also shown that getting five hours or less of sleep can increase the likelihood of developing chronic disease.

Dr. Tony Cunningham, clinical psychologist and director of the Center for Sleep and Cognition in Boston, says it isn’t as simple as getting the “right” number of hours of sleep. In a conversation with CNN, he explained why.

Sleep quality matters as much as sleep time

Many people focus on how many hours they sleep while overlooking sleep quality, which can be even more important.

“There are two different things going on in our bodies that dictate both the type of sleep that we’re getting and the quality of sleep that we’re getting, and that is our sleep pressure and circadian rhythms,” said Cunningham, who is also an assistant professor of psychology at Harvard Medical School.

Sleep pressure, also known as sleep drive, builds up the longer you stay awake and decreases while you sleep. It’s what causes you to feel increasingly tired after being awake for a long time.

“It’s just like eating,” Cunningham said. “The longer it’s been since you’ve eaten, the hungrier you get.”

To get a good night’s sleep, it’s best to go to bed when you’ve built up enough sleep pressure.

Your circadian rhythm is your body’s internal clock. While external factors such as light can influence it, the rhythm itself is guided by the brain.

“The circadian rhythm can fluctuate and send either sleep-promoting or wake-promoting signals throughout the day,” Cunningham said. “If you’ve ever pulled an all-nighter and gotten a second wind in the middle of the night, that’s your circadian rhythm kicking in.”

For high-quality sleep, sleep pressure and circadian rhythm need to work together. Abrupt changes or an irregular sleep schedule can disrupt this balance and lower sleep quality.

One effective way to improve sleep quality is to wake up at the same time every day. According to Cunningham, this can be more impactful than going to bed at the same time, since going to bed when you’re not sleepy isn’t always helpful.

Once you establish a consistent routine, your body will naturally begin to seek its optimal sleep time.

How many hours of sleep do you really need?

Advising people to aim for seven to nine hours of sleep is reasonable, but it’s important to remember that this is an average.

“That does not mean that every single person needs eight hours of sleep,” Cunningham said. “Some people truly only need five or six hours to function optimally, based on their biology and physiology.”

The opposite is also true. “There are people who need nine, 10 or even 11 hours of sleep per night,” he added.

To figure out your personal sleep needs, Cunningham recommends two steps.

First, keep a consistent bedtime that you’re confident will allow you to fall asleep within 20 to 30 minutes. Your bedtime should be when you feel sleepy, not just tired.

If you can’t fall asleep within that 20-to-30-minute window, it likely means you haven’t built up enough sleep pressure. In that case, it’s better to get out of bed and do something calming, such as taking a warm bath or meditating with dim lights, until you start to feel sleepy.

The second step is to find a period in your life when you can sleep until you wake up naturally, without an alarm.

“Hide your clocks, block out light with curtains, use noise machines, wear an eye mask — anything that removes time cues,” Cunningham said. “Then go to sleep and wake up naturally.”

Not everyone can do this, but students on holiday breaks or people taking time off work may find it a worthwhile experiment, especially if they’re concerned about their sleep.

“The first few days, you’ll probably sleep longer than usual as your body catches up,” Cunningham said. “It’s not uncommon to sleep until late morning during that period.”

After a few days, you’ll know you’ve found your ideal sleep duration when you wake up naturally at about the same time for several days in a row, without alarms or external cues.

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