Six hours of sleep

1

What do two weeks of six-hour nights do?

Mental performance slipped further each night. Two weeks at six hours or less matched up to two nights without sleep.

The cost of chronic short sleep is often assumed to be small. In this experiment, the cost kept adding up for two weeks.¹

A 2003 report from the University of Pennsylvania in Philadelphia described 48 healthy adults who lived in a sleep lab for two experiments. They were in their twenties and thirties, and they stayed under constant monitoring.¹

In the first experiment, each person was randomly given a fixed time in bed every night: four, six or eight hours. This went on for 14 days in a row, and all other sleep was prohibited.¹

A second experiment served as a yardstick: people went three nights without any sleep.¹

Both the four-hour and six-hour groups developed deficits on every mental task the team tested. The deficits were cumulative: they grew night by night, and more with the shorter nights. By the end they were as large as after up to two nights with no sleep.¹

A statistical model gave a simple rule behind this. Lapses are moments when a response comes more than half a second late. They rose almost in a straight line with the total time people spent awake beyond about 16 hours, however the sleep was lost. The authors suggest sleep debt may be best understood as extra wakefulness that carries a cost for the brain, and the cost adds up.^1,2

The people were healthy and young, and napping was not allowed. The tests were lab tasks, so they cannot say how a real working day would go.¹

Six hours is what a midnight bedtime and an alarm at six leave you. That was the dose here, taken night after night.¹

2

Would you notice the damage?

Barely. Sleepiness ratings jumped at first, then rose little, and did not tell the six-hour group from the four-hour group.

Performance kept falling, but feeling did not follow. People rated themselves sleepier after the first short nights, then only a little more over the following days. The ratings did not clearly separate the six-hour and four-hour groups, whose mental performance differed.¹

The authors read this as people being largely unaware of their growing deficits. They suggest that may be why the cost is so easily missed.¹

Sleep itself did not mirror the decline. A slow brain-wave signal in sleep is thought to track the body's need for sleep. It changed at first, then barely moved across the 14 restricted nights. Waking performance kept falling.¹

So how sleepy you feel on the fifth short morning may say little about how well your attention is working.¹

3

Do other studies find the same?

Broadly. A review of 61 studies found impaired thinking, but a one-week study saw performance level off at five and seven hours.

A 2017 review pooled 61 experimental studies in which researchers cut people's sleep for a short time. Overall, the cuts impaired thinking. The effect was largest for sustained attention, holding focus on one task. It was smaller for executive functioning, the planning and flexible side of thinking, and smallest for long-term memory. For some skills, such as multitasking and impulsive decision-making, the evidence was too thin to say.³

Not every study saw a steady slide. At the Walter Reed Army Institute of Research, 66 volunteers spent a week on fixed nights in bed. The nights ranged from 3 to 9 hours.²

In the shortest group, lapses climbed steadily all week. In the middle groups, speed dipped and then seemed to settle at a lower level, and lapses rose only in the 5-hour group. The longest group stayed at baseline.²

The authors took this as a sign that the brain adapts to mild or moderate cuts, though at a lower level of performance. The two lab studies differ in length and doses, so together they do not settle whether the slide ever stops.^1,2

The two lab studies ran for a week or two. Whether years of six-hour nights keep costing you, or settle, is not something they can tell.^1,2

4

Can you pay it back with a good night?

Not in one night. After five nights at four hours, even ten hours in bed left some deficits.

A 2010 study by the same team tested one night of recovery in 159 healthy adults. Most spent only four hours a night in bed for several nights. Each of those was then randomly given one more night, anywhere from no sleep to ten hours in bed.⁴

Lapses, sleepiness and fatigue improved the more people slept. But after ten hours in bed, about nine hours of actual sleep, some deficits were still there.⁴

In the week-long study, three recovery nights of eight hours did not lift the middle groups back to baseline: speed stayed at its lower level. The shortest group bounced back after the first night, but only partly.²

Both studies tested recovery over a few days. Neither says how a shortfall repays itself over weeks.^2,4

A long lie-in after a short week is one recovery night, the dose tested here.⁴

5

Is everyone hit the same?

No. In 21 adults kept awake for 36 hours, the hit differed a lot between people but was stable within each person.

The Pennsylvania team asked whether some people are hit harder than others. They studied 21 healthy adults. Each stayed awake for 36 hours on three separate visits.⁵

Before each visit, people either limited sleep to six hours a night or spent 12 hours a night in bed for a week, in random order.⁵

Some people were hit much harder than others, and the same people were hit hard each time. The gap between people was large next to the effect of the earlier week of short sleep. That week gave about four hours a day less sleep than the long-sleep week. Baseline performance did not explain it.⁵

The differences clustered on three distinct fronts: how sleepy and low people felt, how well they thought, and how well they held attention.⁵

This was a test of 36 hours awake, not of six-hour nights, so it shows that people differ, not by how much in your own short nights.⁵

So the lab averages are a starting point. You could lose more or less than they did, and the study found nothing simple that predicts which.⁵

6

What does this mean for you?

Six-hour nights have a measurable cost that feeling does not show. The studies tested short runs of nights in healthy adults.

1. Count the nights. The cost built across a run of short nights, so a tally of them says more than one morning's mood.¹
2. Know the line. The study's model counts only wakefulness beyond about 16 hours a day, which leaves about eight hours for sleep. It is a fitted model in young adults, not a recommendation, and How much sleep you need has its own article.¹
3. Give recovery more than a night. One long night did not fully repay a run of four-hour nights in the lab.⁴
4. Expect to differ. Some people lose more than the averages show, and some lose less.⁵