Too much sleep: when long nights are a warning sign
Walker and Attia both address oversleeping, noting that very long sleep is often a marker of poor sleep quality, illness or depression. It corrects the idea that more time in bed is always better.
People who sleep very long do, on average, die sooner than people who sleep seven to nine hours. The most likely reason is that long sleep is a sign of something else, not the cause. A regular need for ten or more hours is worth treating as information about your health or your sleep quality. For most people the real problem is still too little sleep.
Peter Attia, a physician, describes the data as a curve. Mortality is higher among people who sleep little, falls as sleep increases, and ticks up again among very long sleepers. He says "there is no question that hyper sleep has been associated with poor outcomes." The debate is about why. Historically the upturn was explained by sick people sleeping more, and he thinks that covers most cases, though he notes some emerging evidence of a downside beyond it. He calls the question controversial in the sleep literature.
Matt Walker, a sleep scientist, puts the upturn past about 10 to 11 hours and gives two explanations that do not involve sleep doing harm. One is illness: the long sleep reflects a sickness that sleep could not overcome. The other is quality: people whose sleep is broken stay in bed longer to make up for it. He measures quality by sleep efficiency, the share of time in bed spent asleep, and puts the healthy level at about 85 percent or higher. He does not rule out that too much sleep is possible, in the way too much food or water is.
Walker adds a third explanation from depression, where oversleeping is most established. The studies asked patients when they went to bed and when they got up, and assumed that "if you're in bed for longer, then you're sleeping for longer". When researchers asked when people fell asleep and woke, the effect was much weaker. Depressed people stay in bed because they do not want to face the world, while their actual sleep is usually shorter and broken. Timing matters too: a sleep schedule out of line with the body clock predicts depression.
Andrew Huberman supplies the baseline. Most people need at least six and ideally seven to eight hours, and children, teenagers and people under heavy strain need more. His test is daytime sleepiness.
The two experts agree on the curve and on illness as the leading explanation, and both leave open whether long sleep in a healthy person does harm. None of this applies to catching up after real deprivation, and it is not a reason to cut sleep.
What to do with it: separate time in bed from time asleep. Check for waking in the night. If you sleep long and still doze off during the day, treat that as a sign something is off. Look at whether your schedule fits your body clock. After a bad night, keep your normal schedule instead of sleeping in. Keep naps to about 20 minutes if your nights are already full. Aim for seven to nine hours of good sleep.
If long sleep is new, persistent, or comes with low mood or daytime exhaustion, see a doctor. If a change in sleep has come with a change in mood, in you or someone close to you, contact a doctor or a crisis line.