Is six hours enough, and can you recover from years of short sleep?

Six hours can be enough for some people, but it is not a safe target for most, and years of short sleep can be recovered from if you fix your sleep now. That is the combined answer from Andrew Huberman and Matthew Walker, who agree on how to judge your sleep and differ on the number.

Huberman says sleep need is individual and the test is how you function during the day. If you feel reasonably good, have the energy and focus to do what you need to do, and at most want a 10- to 30-minute afternoon nap, he would not worry about five to six hours. He still sets a goal: for 95% of people, at least six hours and ideally seven or eight. At four or five hours he doubts you can get a balance of slow-wave and REM sleep.

Walker sets the bar at seven to nine hours for most adults, and lists what six hours costs. People averaging six hours or less are almost three times more likely to catch a cold, and one week of six-hour nights changed the activity of 711 genes. Waking two hours early can cost 60 to 80 percent of your REM. He stresses that one bad night does not doom anyone; the concern is chronic short sleep.

Both use Walker's four-part test, QQRT: quantity, quality, regularity and timing. Quality means continuous sleep, with 85 percent or more of your time in bed spent asleep. Regularity means keeping bed and wake times within 30 to 60 minutes. Timing means fitting sleep to your chronotype.

On recovery, Huberman is direct: "It's very clear that the brain can recover." He told a man who slept five hours a night for 15 years that substantial damage was unlikely, with one exception, a whole life of short sleep addressed only late. The article notes he cites no study following long-term short sleepers, so this rests on his judgment.

They disagree in three places. Walker's range starts at seven and Huberman's at six. Walker warns that you do not realize how impaired you are, much like a drunk driver, which undercuts a test based on how you feel. And Huberman believes 10 to 30 minutes of non-sleep deep rest can restore some of what a short night takes, while Walker's position, in Huberman's words, is that "you need your sleep, period."

What to do with it: run the daytime checks. Would you sleep past your alarm? Can you function with good mood and clear thinking before 11 a.m. without caffeine? Are you having lapses of attention? Score all four parts of QQRT, not only the hours. Fix regularity first, and set a bedtime alarm as well as a wake-up alarm. Protect the end of the night, avoid afternoon caffeine and evening alcohol, and treat tracker scores as estimates. If you were a short sleeper for years, start now, drop the guilt and aim to get it right 80 to 90 percent of the time. If you work shifts, hold one schedule for at least two weeks at a time and take whatever sleep you can get.

If daytime sleepiness persists, or you snore heavily or stop breathing in your sleep, see a doctor or a sleep specialist.