Is Six Hours of Sleep Enough? Two Weeks of It Matched a Full Night Awake

After two weeks of six hours of sleep a night, the brain was in the same state as after one full night without sleep. That is the result of a sleep restriction experiment in the United States, published in the journal Sleep in 2003.

In a Brain Mondo short video, I introduced this experiment in about 40 seconds. In this post I write about the details that did not fit in the video, and about what the study does and does not tell us.

Video: Sleep facts: Is six hours of sleep enough? (Japanese)

The video is in Japanese.

What the experiment was

The study was run by a team at the University of Pennsylvania led by Van Dongen and Dinges. The participants were 48 healthy adults aged 21 to 38. All of them stayed in a research facility, where their behaviour and physical state were monitored the whole time.

Each participant was assigned 4, 6 or 8 hours a night in which they were allowed to be in bed, and kept to it for 14 days. Naps were not allowed. Another group went through three nights in a row with no sleep at all. Every group also had three days before the experiment began and three days of recovery afterwards.

Throughout, participants took cognitive tests again and again every day. The main one was a simple reaction test: press a button as fast as you can when a number appears on the screen. Any response slower than half a second was counted as a “lapse”. There were also a test of matching symbols to digits and a test of continuous addition and subtraction.

One thing to keep in mind is that six hours was the time allowed in bed. You do not fall asleep the moment you lie down, so the actual sleep was shorter. People who say “I get six hours” are often counting from getting into bed to getting up. Counted that way, they are in almost the same condition as the six-hour group in this study.

Result: a slow, steady decline

Over the 14 days, the 4-hour and 6-hour groups got gradually worse on every test. The decline matched how much sleep had been cut, and the 4-hour group fell faster. As the video says, the 8-hour group stayed about the same, with no decline of this kind.

The paper’s conclusion is this. Restricting sleep to 6 hours or less a night produced performance deficits equivalent to up to two nights of total sleep deprivation. By the end of the two weeks, the 6-hour group had fallen to about the level of one night without sleep, and the 4-hour group to about the level of two nights.

“Pulling an all-nighter” sounds like something special. Six hours of sleep, for many working adults, is just an ordinary night. Stack two weeks of ordinary nights and you have the head of the morning after an all-nighter.

They did not notice

This experiment also looked at how the participants felt, not only at their test scores.

When participants rated their own sleepiness, it rose in the first few days of restriction, and after that it rose only slightly. On top of that, there was no clear difference in felt sleepiness between the 6-hour and 4-hour groups. Their scores were dropping day by day while their own sense of things stayed almost flat.

The authors write that participants were “largely unaware” of their increasing cognitive deficits. They go on to say this may be why the effects of chronic sleep restriction are often seen as harmless.

Many people have the feeling that “I’ve got used to six hours” or “my body is on this rhythm now”. The natural reading of this experiment is that what got used to it was the feeling of sleepiness. The brain’s performance did not.

The number 15.84 hours

The paper has one more interesting analysis. The team pooled the sleep restriction groups and the total sleep deprivation groups and looked at how time awake related to the number of lapses.

Whatever the form of sleep loss, lapses rose almost in a straight line as the time spent awake beyond about 15.84 hours a day piled up. 15.84 hours is 24 hours minus about 8.2 hours. In other words, the “extra waking time” left over when you do not get about 8 hours of sleep accumulates like a debt, and the total of that debt decides how well the brain works.

The authors describe sleep loss as an accumulation of “additional wakefulness” that carries a neurobiological cost. Six hours of sleep is a debt of about two hours a day. Fourteen days of that comes close to one full night awake. Seen in numbers, the result is not so mysterious.

Can you catch up on the weekend?

So can you make up on the weekend what you cut during the week? In the same year, 2003, a different American team (Belenky and colleagues) looked at this, including recovery.

In their study, 66 participants spent 7 days with 3, 5, 7 or 9 hours a night in bed, followed by 3 days of 8 hours in bed. In the 5-hour and 7-hour groups, response speed dropped at first and then levelled off at the lower level. During the 3 days back on 8 hours, it barely recovered from that level. The 3-hour group recovered a lot after the first night of recovery sleep, but stopped at about the same level as the 5-hour and 7-hour groups.

It is worth remembering that response speed dropped even in the 7-hour group. The authors suggest that the brain adapts to sleep loss and stabilises performance at a lower level, and that this adaptation lasts for several days after a return to normal sleep.

What these two studies tell us is that the effects of sleep loss do not simply vanish after one good night. Wiping out the week’s lost sleep with a weekend lie-in looks difficult.

How much sleep is enough?

Japan’s Ministry of Health, Labour and Welfare, in its “Sleep Guide for Health Promotion 2023”, recommends that adults get the sleep they need, with 6 hours or more as a guide. A 2015 joint consensus statement from the American Academy of Sleep Medicine and the Sleep Research Society says adults should regularly sleep 7 or more hours a night.

How much sleep a person needs varies. Even so, since the six-hour group in this study had six hours in bed, one way of thinking about it is to aim for 7 to 8 hours in bed.

Using it in daily life

Here are three ways to apply this study to daily life.

• When mistakes pile up, suspect sleep first. Your own sense of sleepiness is not reliable. In a week full of slips, before blaming your attention, try recalling how much you slept over the past week or two. The cause is sometimes right there.
• Write down when you go to bed. If you cannot judge by how sleepy you feel, you have to judge by the clock. Many people who write down when they went to bed and got up for a week find that they sleep less than they thought.
• Save important decisions for well-rested days. Work where a miss is not acceptable, such as checking a contract or going through detailed figures, is safer on a day when you have slept enough.

Brain Mondo is a series of daily short videos about the brain, each based on a research paper. BrainDojo is an app for training calculation, memory, attention and more in a few minutes a day, and the once-a-day Brain Dojo Check gives you scores for processing speed, inhibitory control and short-term memory. Comparing your scores on a short-sleep day and a well-rested day might make this experiment feel a little closer to home.

References

• Van Dongen HPA, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep. 2003;26(2):117-126.
• Belenky G, Wesensten NJ, Thorne DR, et al. Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study. J Sleep Res. 2003;12(1):1-12.
• Watson NF, Badr MS, Belenky G, et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep. 2015;38(6):843-844.
• Ministry of Health, Labour and Welfare (Japan), “Sleep Guide for Health Promotion 2023” (in Japanese)