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 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."
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.
Sleep health, as defined by Buysse, is a complex, multidimensional framework encompassing six key dimensions: regularity, satisfaction, alertness, timing, efficiency, and duration. 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 to determine whether a person had a good night's sleep. Even so, one of the best practices to determine sleep health, says Lucey, may still be simply asking a person, "How did you sleep?"
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.
But even that range doesn't tell the full story. 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," says Buysse.
Sleep also seems to increase "brain washing," a process in which fluid flows through the brain to remove waste. The brain's glymphatic system is thought to help clear waste, including proteins associated with Alzheimer's disease. Chronic shortened sleep is also linked to increased risk of cardiovascular disease, dementia, and premature death.
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. "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 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."
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. Life is unpredictable, and trying to catch up on sleep the next day is fine, but it's important to get back into routine as soon as possible.
The setup for a good night's sleep starts early. Morning light exposure, exercise, and tapering off caffeine throughout the day can help with sleep. At bedtime, a steady routine and a cool, dark, quiet room 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.
Despite decades of research, scientists say sleep remains one of biology's biggest mysteries. The brain during sleep is the same brain that functions during the day — it's just operating in a different mode. That perspective could reshape how scientists interpret links between sleep and disease, pointing to sleep not as a passive shutdown state, but another mode of brain activity that researchers are only beginning to understand fully.