How little sleep can you get away with?

Until about 15 years ago, one common theory was that if you slept at least four or five hours a night, your cognitive performance remained intact; your body simply adapted to less sleep. But that idea was based on studies in which researchers sent sleepy subjects home during the day — where they may have sneaked in naps and downed coffee.

Enter David Dinges, the head of the Sleep and Chronobiology Laboratory at the Hospital at University of Pennsylvania, who has the distinction of depriving more people of sleep than perhaps anyone in the world.

In what was the longest sleep-restriction study of its kind, Dinges assigned dozens of subjects to three different groups for their 2003 study: some slept four hours, others six hours and others, for the lucky control group, eight hours — for two weeks in the lab.

Psychomotor vigilance

Every two hours during the day, the researchers tested the subjects’ ability to sustain attention with what’s known as the psychomotor vigilance task, or PVT, considered a gold standard of sleepiness measures. During the PVT, the men and women sat in front of computer screens for 10-minute periods, pressing the space bar as soon as they saw a flash of numbers at random intervals. Even a half-second response delay suggests a lapse into sleepiness, known as a microsleep.

Those who had eight hours of sleep hardly had any attention lapses and no cognitive declines. What was interesting was that those in the four- and six-hour groups had PVT results that declined steadily with almost each passing day. Though the four-hour subjects performed far worse, the six-hour group also consistently fell off-task. By the sixth day, 25 per cent of the four-hour group was falling asleep at the computer. And at the end of the study, they were lapsing fives times as much as they did the first day.

The six-hour subjects fared no better on a test of working memory in which they had to remember numbers and symbols and substitute one for the other. The same was true for an addition-subtraction task that measures speed and accuracy. All told, by the end of two weeks, the six-hour sleepers were as impaired as those who, in another Dinges study, had been sleep-deprived for 24 hours straight.

So, for most of us, eight hours of sleep is excellent and six hours is no good, but what about if we split the difference?

While Dinges’s study was under way, his colleague Gregory Belenky was running a similar study. He purposely restricted his subjects to odd numbers of sleep hours — three, five, seven and nine hours — so that together the studies would offer a fuller picture of sleep-restriction. Belenky’s nine-hour subjects performed much like Dinges’ eight-hour ones.

Of course our lives are more stimulating than a sleep lab: we have coffee, bright lights, the social buzz of the office, all of which work as ‘countermeasures’ to sleepiness. They can do the job for only so long, however. Belenky said “You don’t see it the first day. But you do in five to seven days. Unless you’re doing work that doesn’t require much thought, you are trading time awake at the expense of performance.”

And it’s not clear that we can rely on weekends to make up for sleep deprivation. Dinges is now running a long-term sleep restriction and recovery study to see how many nights we need to erase our sleep debt. But past studies suggest that, at least in many cases, one night alone won’t do it.

Not every sleeper is the same, of course: Dinges has found that some people who need eight hours will immediately feel the wallop of one four-hour night, while other eight-hour sleepers can handle several four-hour nights before their performance deteriorates.