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How much sleep do you need? Some people function beautifully on three hours each night. Quite a few others, particularly people who are ill, need eight or nine—or more. It takes time to rebuild the body, and different people require different amounts of rest.
When the American Association of Sleep Medicine was tasked to give a “national” answer of how much sleep people should get, they recommended a guideline of seven-to-nine hours a night. More recent studies suggest that seven hours may be the sweet spot for health and performance.
A recent study by Femke Rutters et al. looked at 788 healthy men and women between the ages of 30 and 60. They studied the relationship among insulin, potential diabetes, and heart disease, and they also looked at sleep.
For the men, but not the women, sleeping more than seven hours, or less than seven, decreased insulin sensitivity and other factors related to mature-onset diabetes. Many studies have shown that sleep disruption or partial deprivation disrupts glucose metabolism. But the seven-hour figure stands up here, and it has been found in other studies, where the number was true for men and women. Glucose physiology is critical to weight control, which is no real surprise—it’s the main source of fuel in the body.
This doesn't mean seven hours of bed is all you need. The study measured an indirect marker for sleep, not sleep time. Even perfect sleepers manage, at best, 95% to 98% of their time in bed actually asleep. If you try to get seven hours of sleep to see if you feel more alert and productive, you probably need to spend seven and half or seven and three quarter hours in bed. Sleep efficiency—the time asleep divided by the time spent in bed—decreases gradually with age. Many physically healthy folks over the age of 70 have a sleep efficiency of 85% or less.
Total sleep time variability across the globe does not vary that much in industrialized countries. But working Americans typically get six and a half to six and three quarters hours of sleep a night. Quite a few of these people feel they function quite well on that amount. However, for many people this means prolonged partial sleep deprivation over years and decades. That takes a toll on the total population. The most obvious manifestations are subtle degradation in productivity, more insulin insensitivity and diabetes, and greater weight gain.
The best time to sleep is at night. Humans are daytime animals. The quarter of the American population that does shift work knows this very well. There is a whole industry engaged in trying to get shift workers optimal sleep and rest. Disrupting biological clocks also disrupts people’s health. Some of the adaptations shift workers use are naps on the job (which sometimes get them fired); naps at biologically appropriate times on both working and non-working days; and long, recovery sleeping periods on days off. Even with the best adaptations, shift work worsens long term health.
A society that runs around the clock is better for machines than people. Our work and social patterns are so pervasive that many people start viewing themselves as machines. This can really hurt your health, especially your ability to regenerate yourself. One common adaptation people use is short naps. Naps as short as six minutes have been shown to improve alertness and productivity, while naps between 10 and 30 minutes appear to disrupt nighttime sleep less than longer naps.
Another major adaptation is the use of techniques that can improve sleep efficiency, so you get as much sleep as you can during your time in bed. A cool, calm, comforting sleep environment helps. Walking in the morning, when it's light, can also be beneficial. There is something about morning light that improves mood, mental sharpness, and our ability to sleep—some of this probably comes from the light resetting our biological clock. Another factor to consider is physical activity and exercise of almost every type. People who are fit usually experience greater sleep efficiency. They may also experience more deep sleep and REM sleep, which aid learning and bodily regeneration.
The bottom line is that sleep needs are uniquely biological to every human being. Some of us only get a few hours of sleep each night, but those people are in the minority. It looks like the national recommendation for seven hours of sleep—not time in bed, but sleep—may be a helpful guideline for adults who want to live long and not become diabetic or overweight. Getting that amount of sleep means spending more than seven hours in bed, having a regular biological clock (aided by going to bed and getting up at regular hours), and staying reasonably fit.
What you do is what you become. Sleep is like food. If you get the right amount, in the best ways for your body, you function better.
