Inside the science of great sleep
The old advice to simply get eight hours misses a bigger picture, researchers say. New studies reveal that timing, consistency, and sleep quality all play a major role in health.
For decades, the advice was simple: get eight hours of sleep a night to feel refreshed the next day. The guidance was tidy, easy to remember, and rooted in a 19th-century labor reform marketing campaign that divided the day into equal parts: eight hours for work, eight for leisure, and eight for rest.
But sleep researchers now say focusing solely on the number misses the bigger picture. Studies suggest fragmented, start-and-stop sleep can leave people just as fatigued as sleeping too little. Moreover, scientists increasingly understand sleep not as passive downtime, but as a period of intense biological activity tied to brain function, metabolism, immune health, and emotional regulation.
“Sleep is more complicated than first meets the eye,” says Daniel J. Buysse, a professor of psychiatry and endowed chair in sleep medicine at the University of Pittsburgh. “I think when we try to define what is good quality sleep or how sleep promotes health, we have to quickly get beyond just thinking about the duration.”
So, if sleep health is not defined by eight hours of shut-eye, what actually matters most?
The total number isn’t the whole story
Once regarded as a passive time for the human brain to power down, sleep is now understood as a period of intense neurological activity and critical maintenance. Research suggests sleep helps regulate immune function, supports metabolic health, and allows the brain to process information and restore itself overnight.
“Good sleep is essential to good health,” wrote Buysse in a 2014 paper published in the journal Sleep, arguing that sleep should be understood not just by disorders. Sleep health, as defined by Buysse, is a complex, multidimensional framework encompassing six key dimensions: regularity, satisfaction, alertness, timing, efficiency, and duration.
(When you go to bed may matter more than how long you sleep.)
Experts say sleep is a daily behavior—like diet and exercise—that can be shaped to support a person’s well-being.
However, “good” sleep is highly individualized and often difficult to measure, says Brendan P. Lucey, professor of neurology at Washington University. No blood test exists, for example, to determine whether a person had a good night’s sleep. To measure sleep quality, researchers use a range of tools, from wearable multi-sensor monitoring devices to lab-based studies and self-reports.
Even so, one of the best practices to determine sleep health, says Lucey, may still be simply asking a person, “How did you sleep?”
The brain’s night shift
Part of the reason sleep matters so much is what happens beneath the surface. A large body of research suggests that both too little and too much sleep are associated with worse cognitive performance and a higher risk of decline over time. There’s a “sweet spot” in the middle, says Lucey, usually around seven to eight hours of sleep, where thinking, memory, and focus work best.
(There's a better way to wake up. Here's what experts advise.)
But even that range doesn’t tell the full story. Using a multidimensional framework of sleep health shifts the focus beyond total sleep time, says Buysse. If a person can function well with five hours of sleep, and the sleep session is uninterrupted and regularly timed every night, then “that’s not a person I would try to get to sleep longer,” he says.
Sleep also seems to increase “brain washing” or sloshing, a process in which interstitial fluid flows through the brain parenchyma into the cerebrospinal fluid to remove waste, says Lucey. In this process, the brain’s glymphatic system is thought to help clear waste, including amyloid and tau, which are associated with Alzheimer’s disease.
The cost of poor sleep
Chronic shortened sleep is also linked to a range of negative health outcomes, including increased risk of cardiovascular disease, dementia, and premature death. Studies have found that people who regularly sleep fewer than six hours a night are more likely to develop conditions such as hypertension, obesity, and type 2 diabetes.
Severe sleep loss has also been associated with changes in inflammation and glucose regulation.
(How do a few nights of bad sleep become insomnia?)
But sleep experts say one of the biggest problems can begin when people become anxious about sleep itself. Sometimes patients arrive at Buysse’s office in a panic, citing research linking poor sleep to negative health outcomes. They start chasing a number, trying harder and harder to force themselves to sleep longer.
“I can tell you that the one thing that doesn't work for treating insomnia is trying harder [to sleep longer],” says Buysse. “It just doesn't work.”
Instead, Buysse tries to shift the focus onto a more controllable variable, such as optimizing uninterrupted sleep. “Rather than immediately saying, ‘Oh, I have to sleep longer,’ Let's sleep better,” he says. “In a more consolidated way, and eventually that might even lead to longer sleep.”
Building better sleep
The ideal sleep situation, says Marie-Pierre St-Onge, a professor of nutritional medicine at Columbia University, is to get at least seven hours every night by going to bed and waking up at the same time—consistently.
But life is unpredictable, and sleep can be elusive. In those cases, trying to “catch up” on sleep the next day to feel refreshed is fine, but it’s important to get back into routine as soon as possible, says St-Onge.
That’s because humans can get by on less than ideal sleep for a time, but the side effects tend to accumulate over time.
“The amount of stress you can handle is affected by sleep,” says A.J. Schwichtenberg, associate professor at Purdue University and director of the Sleep and Developmental Studies Laboratory. “And anyone who has ever had an all-nighter or traveled for extensive periods will tell you that emotional regulation very quickly takes a toll.”
The setup for a good night’s sleep starts early. Morning light exposure, exercise, and tapering off caffeine and other stimulants throughout the day can help with sleep, says Schwichtenberg. At bedtime, a steady routine and a cool, dark, quiet room can help carry it through.
Diets rich in fruits, vegetables, and fiber—and lower in saturated fat and added sugars—are associated with deeper, more stable sleep with fewer disturbances, says St-Onge, author of Eat Better, Sleep Better. Some studies suggest that foods like walnuts, cherries, and tomatoes may support sleep, St-Onge adds.
What scientists still don’t know
Despite decades of research, scientists say sleep remains one of biology’s biggest mysteries.
“This is the wonderful enigma of sleep,” says Schwichtenberg. Part of the challenge in the research world is that the sleeping brain is often treated differently from the awake brain. But Schwichtenberg’s work and her forthcoming book suggest that assumption may be misleading.
The brain during sleep is the same brain that functions during the day, says Schwichtenberg. It’s just operating in a different mode.
That perspective could reshape how scientists interpret links between sleep and disease. Studies have associated poor sleep with conditions like Alzheimer’s disease, but the relationship is complex. Scientists don’t think it’s a one-way relationship—sleep problems and Alzheimer’s likely feed into each other, as part of the same underlying changes in the brain.
The theory points to a broader shift in research. Sleep may not be a passive shutdown state at all, but another mode of brain activity—one that researchers are only beginning to understand fully.
