Gut Check is a periodic look at health claims made by studies and by newsmakers. We ask: Should we believe this?

The Claim: Healthy adults need eight hours of sleep each night, preferably uninterrupted, and children need a lot more.

The Backstory: The eight-straight-hours ideal dates back to the Industrial Revolution, when reformers seized on the divisibility of 24 into three equal periods to recommend eight hours of work, eight of rest, and eight of sleep. That kept factory owners from demanding 14 hours of work, but it had no scientific basis. Things started to get more rigorous in the 1950s, with studies over the next 60 years linking inadequate sleep to chronic conditions such as depression, obesity, type 2 diabetes, hypertension, stroke, and cardiac arrhythmias.

That scared insomniacs into thinking: Great, not only am I staring at the ceiling at 3 a.m., but I’m giving myself diabetes. But that research has problems.

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Even if poor sleepers are more likely to have, say, hypertension, “you can’t infer causality from that,” said Dr. Allan Pack of the University of Pennsylvania medical school, who is president of the Sleep Research Society. Maybe disorders cause poor sleep, rather than vice versa. Or maybe a third factor causes both poor sleep and its associated disorders.

Although people who are awakened in a sleep lab after just four or five hours of shut-eye every night for a week typically experience insulin resistance and a fall in levels of the appetite-quelling hormone leptin, that isn’t definitive either. Being awakened after five hours is not the same as waking naturally after six or seven hours. For one thing, the wake-up call might come during the deepest part of your sleep cycle. When you’re rudely yanked into consciousness, you feel like death warmed over and might pay a price physiologically, whereas if you’d slept even a few minutes longer you might have reached light, Stage 1 sleep, when waking feels natural and doesn’t exact a physiological toll. Also, there are almost no data “on people who are chronically short sleepers,” said Pack. “It’s possible that there are adaptations to short sleep” that allow the six- and sevenhour crowd to function just fine.

Second Take: Such concerns have undermined the eight-hours-straight rule, so much so that in 2015 the American Academy of Sleep Medicine officially said that seven hours is fine. Perhaps the most significant realization is that “split sleep,” when you’re awake for an hour or two between two solid stretches of sleep, “is just as good as uninterrupted sleep,” said sleep researcher Fred Turek of Northwestern University. Another change is the recognition that “there is a lot of individual variation in the amount of sleep people need, which is partly genetic,” said Pack. “You need the amount of sleep that keeps you alert during the day.”

The Takeaway: If you awaken after three or four hours and then later fall asleep again for a stretch, don’t assume it’s unhealthy insomnia: Interrupted sleep is common and just as good as the continuous variety. And if you average seven hours, you’re not necessarily falling short; if you’re functional the next day, you got enough sleep.

This article was originally published on Nov. 25, 2015.

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