Contrary to traditional beliefs, there is no proven link between sleep deprivation and illness, but studies show sleeplessness causes accidents.
Claudio Stampi, director of sleep research at the Institute for Circadian Physiology in Boston, studied 20 sailors who on long solo voyages habitually went for days or weeks with four hours sleep or less each day. They reported they did not have any increase in colds or infections.
Five other volunteers in Stampi's lab who got just three to four hours sleep each day for a month also did not become ill.
On the other hand, James Krueger, a sleep researcher at the University of Tennessee in Memphis, has found that being sick makes people sleepy because fighting off bacteria or a virus triggers the production of an immune system component that also induces drowsiness.
So although there is no proof that too little sleep lowers resistance, there is evidence that we are meant to get more sleep to help us combat illness.
The consequences of sleep deprivation go beyond feeling tired. Mental performance suffers. Fatigue darkens mood and impairs concentration, memory and decision-making ability. And the longer you skimp on sleep, the more the effects build up.
Sleep researchers say this is the reason so many major industrial accidents happen late at night.
"All available scientific data indicate people are most likely to make mistakes between midnight and 6 a.m.," said Merrill Mitler, director of research for sleep disorders at the Scripps Clinic in La Jolla.
The greatest toll from sleeplessness may be auto accidents. A 1990 study by the National Transportation Safety Board of 182 fatal trucking accidents concluded that driver fatigue was the leading cause (31%), ahead of drugs, alcohol, worn tires and bad roads.
Likewise, a 1988 study of more than 6,000 fatal single-car accidents found that more than 60% occurred between midnight and 6 a.m.
"Sleepiness--rather than loss of control or errors of judgment caused by drinking or illness--seems to be the most likely cause of these accidents," said David Dinges, a biological psychologist at the University of Pennsylvania and one of the authors of the study.
Using equipment to measure the alertness of airplane pilots and truck drivers, Dinges has observed that the ability to sustain attention is reduced by at least 50% during normal sleep hours.
If an entire night's sleep is lost, attention drops 70%.
"If you go two nights without sleep, you can barely function," Dinges said.
The good news, he said, is that a single night's sleep, one in which you let yourself sleep until your body clock--rather than an alarm--wakes you, is usually enough to regain about 90% of the mental acuity lost through sleep deprivation.
A second full night restores the remaining 10%.