But we stay awake nonetheless. The once-drowsy world of sleep research is currently engaged in a raging debate about whether we sleep more or less than we used to, historically. Diane Lauderdale, a sleep researcher at the University of Chicago, studied the sleep patterns of early-middle-aged adults, and found that average sleep duration ranged from 6.7 hours for white women to 5.1 hours for African-American men – well below the big eight.
Most people overestimate the amount they have slept: Pennsylvania’s Dr. Dinges estimates that 7½ to eight hours in bed snuggles down to 6.3 hours of actual shut-eye.
Even the “circadian larks” – the 20 per cent of the population who are early risers and can resist the effects of sleeplessness better than others – “eventually have problems,” he says.
It is Dr. Dinges and a group of colleagues who demonstrated over the past decade just how impairing sleep loss can be. In one of the most extensive sleep studies ever undertaken, the researchers restricted the sleep of their subjects continuously for two weeks, while administering psychomotor vigilance tests every day. The PVT requires a subject to press a button every time a signal appears on a screen. It’s not a hard test.
The findings were terrifying. The performances of people who got four to six hours of sleep a night declined steadily with every passing day. By the sixth day, a quarter of the six-hours group was falling asleep at the computer, lapsing into five times as many “microsleeps” as they had on Day 1. Working memory, accuracy and speed collapsed.
By the end of the two-week period, the six-hour-a-nighters were as impaired as they would have been staying up 24 hours straight – the equivalent, in performance terms, of being legally drunk.
This is not a trivial result: A two-second delay in response is enough to veer into oncoming traffic. Imagine the effects of six hours of sleep a night for four nights in a row combined with talking on a cellphone while driving (which, of course, no one does any more).
But six hours isn’t much sleep. Another researcher, Gregory Belenky, a psychiatrist who now heads the Sleep and Performance Research Centre at Washington State University in Spokane, performed much the same experiment on subjects who got seven hours sleep a night – more than the average North American. The seven-hour crowd’s scores slowed for three days, then plateaued at a lower rate of performance.
They insisted they were not impaired in any way, despite their drooping PVT scores. The first thing sleep deprivation knocks out, in other words, is your ability to tell if you’re sleep-deprived.
And forget about paying off your sleep debt. “It’ll take several weeks to get performance back,” Dr. Belenky says. “And it isn’t enough to sleep in on the weekends.”
Rattled on the ‘rattler’ shift
A deeper cause of sleeplessness – and a more serious safety issue – is the growing popularity of night shifts, early starts and extended hours. All three are popular cost-saving devices in a recession-conscious, union-wary economy.
Night shifts are particularly nasty because they shorten the light-dark cycle of our circadian clocks – the same effect as the jet lag produced by flying west to east. Most shift workers get off at 6 a.m., drive home to a rising run, fall asleep at 8 a.m., but then wake up at 1 p.m., as their circadian body temperature rises. As a result, Dr. Belenky estimates, the average shift worker gets five hours sleep a night – “which is not enough.”
Which workers are most affected by 24-hour shift work? Precisely the ones you want at their sharpest – train and truck drivers, pilots, police officers, nurses and physicians, financial traders and power-plant and utility workers.
If the night shifts are staggered, they’re even more pernicious. The “rattler” shift brings an air-traffic controller in from 4 a.m. to midnight on Monday, and then earlier and earlier each day until Thursday, when he finishes his shift at 2 p.m. and comes back in to do an all-night shift at midnight. Air-traffic controllers average just over two hours of sleep before such a midnight shift.