Sleep is an incredible physical process. There is an early night surge of growth hormone to restore muscle and a slow drop in blood pressure that gives hearts and blood vessels a break. The recently discovered glymphatic system in the brain works like a washing machine while we sleep, flushing out potentially harmful metabolic debris.
Perhaps that is why good sleep, or the lack thereof, is one of the most evergreen concerns I hear about as a primary care doctor. My patients track sleep hours and sleep scores, upgrade mattresses or turn to white noise machines and a shelf full of supplements. The stress is real: Orthosomnia is an emerging clinical concept in which anxiety about sleep metrics can degrade sleep quality.
We could all benefit from chilling out about sleep. Many of us think we should be sleeping at least eight hours a night, but the evidence for that is shakier than we might assume.
A consistent finding in sleep epidemiology studies is that there is not a magic number below which health suddenly falls off a cliff. Rather, studies that show an association between sleep duration and mortality often find that the lowest risk clusters around seven hours. Risk rises both with not enough sleep and with too much sleep. In fact, a meta-analysis of sleep studies published in the journal Scientific Reports found that the adults with the highest mortality rates were those who slept nine to 11 hours a night. Notably, while risk rose with short sleep, the study did not find much difference in all-cause mortality between six and seven hours a night. While the data is inconsistent and there are some exceptions, several other studies have reached similar conclusions.
Does that mean that getting nine hours of sleep is dangerous? No. The reason that sleeping for a long time is linked to higher mortality is that, on average, people who are sick sleep more. So do many people who are struggling with depression and its mixed influences upon sleep. Once we account for reverse causation, the risk of too much sleep almost disappears.
But that cuts the other way as well: The data show that getting less sleep is strongly correlated with living in poverty, working a job that requires night shifts, psychiatric conditions and chronic pain and illness. At least one study found that when researchers adjust aggressively for such variables, associations weakened substantially.
Sleep quality and the regularity of the routine may matter more than duration. A consistent, good-quality 6.5 hours a night, in which you go to bed around the same time every evening, probably carries less risk than a fragmented, anxious eight-hour routine or a pattern in which sleep varies wildly night to night.
Dr. Michael Perlis, director of behavioral sleep medicine at the University of Pennsylvania, acknowledges that the standard medical recommendation of getting seven to nine hours works for many people but “nowhere near all.” That’s because it “doesn’t take into account sex, age, typical duration of the individual’s wake period, amount of mental and physical expenditure per day, health and basal sleep need,” he told me.
Modern wellness culture tells a story of people increasingly missing their sleep numbers, but the data on whether we are in fact sleeping less than we did 50 years ago is mixed. Studies vary based on how sleep is measured. One survey showed a mild decrease in average self-reported sleep time, dropping by 13 minutes over 27 years. Others find an increase or no change at all.
Some historians believe that before widespread electric lighting allowed us to be up all hours, humans most likely spent more time in bed. But those hours may have been less rigidly consolidated, with greater seasonal variation and more segmented patterns — perhaps an hour or so in the middle of the night to pray, read, talk or do quiet tasks.
In fact, the Hadza people of Tanzania, a group of nomadic hunter-gatherers, sleep an average of 6.25 hours per night. Although in bed for up to nine hours, the Hadza are routinely awake for two or more hours during that window.
All of this is not to undermine sleep as a powerful and healthy habit. Sleep improves performance on tasks, makes driving safer and buoys our mental health. Adequate sleep leads to a stronger immune system, better metabolic and cardiovascular health, and more reliable physical energy throughout the day.
One of the most concerning findings about not getting enough sleep comes from the Whitehall II study, which found that persistent short sleep duration (defined as less than six hours) at ages 50, 60 and 70 was associated with a 30 percent increased dementia risk compared to normal sleep duration (defined as seven hours). However, it remains unclear whether abnormal sleep is a causal risk factor or an early symptom of cognitive decline. The evolving consensus points toward a vicious cycle in which inadequate sleep contributes to dementia, which in turn makes sleep worse.
Nonetheless, I usually find myself going to bed later than I intend. Squeezing in a full day of doctoring, parenting, exercising, cooking and sitting down for a family dinner feels worth it to me. These wedged-in activities reduce my sleep quota, but are also proven to help sustain a healthy body and sound mind.
Hippocrates himself wrote: “Both sleep and insomnolency, when immoderate, are bad.” It turns out this warning about too little and too much sleep holds up surprisingly well in the modern scientific literature over 2,000 years later.
As a family physician, I must balance biological, psychological and social considerations when I care for patients. Here’s what I tell them: Aim for seven hours, accept that some people need more or less to feel rested, be compassionate with yourself about sleep failures and realize that there are times when it’s worth it to shoehorn in a bit more conscious life from a day.
Ryan McCormick is a family doctor who writes about health and medicine in his newsletter Examined.
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