In 1989, when scientists kept rats from sleeping, the rats started to die by the 11th day. By the 32nd day, they were all dead. “We are still clueless as to why the rats died,” said Robert Stickgold, Ph.D., associate professor of psychiatry at Harvard Medical School and an expert on sleep, speaking at internal medicine grand rounds in November. “It happens in rats,” he continued. “It happens in humans.”

For 2,000 years we have understood other basic drives, such as hunger and sex, he said, but the need for sleep remains a mystery. Among early candidates for its apparent benefits, said Stickgold, were restoration and recovery, energy conservation and avoidance of nocturnal predators. “When you try to parse those out,” Stickgold said, “none of those make sense. You end up back at the point that sleep is to cure sleepiness.”

Yet more recent studies have confirmed that a good night’s sleep confers numerous benefits. Without it, the immune system produces fewer antibodies to fight infection. And it is during sleep that the brain consolidates the day’s memories and learning.

“It does matter how much sleep you get,” Stickgold said.