Dear Diedrich,

You’ve got to sleep to know you’re awake.

That may sound like a riddle, but it’s true. Without sleep, your brain begins to shut down and you’re nothing better than a mindless zombie. You may lose track of reality and think you’re sleeping. You may think you’re just having a bad nightmare where you can’t think straight or experience emotion.

But that’s not a very scientific answer. For the expert’s perspective, I spoke with Greg Belenky. He runs the Sleep and Performance Research Center at Washington State University’s Spokane campus.

He gave a tortured answer, so to speak. At least one that sounds like torture.

“All you have to do is look at behavior. Look at somebody who is sleep deprived for 84 hours. They can hardly do anything. They’re docile, easily led. They don’t think very fast. They tend to fall asleep,” he says. “The real limit, which we’ve done in the laboratory, is 84 hours. Going beyond that on the fourth night, you just can’t do it.”

We don’t really know why, though. Theories abound, but prominent ones say sleep helps conserve energy and process memories. The easy answer is we need sleep because we get sleepy.

Of course, nothing is that easy or straightforward. For instance, every animal needs sleep. But elephants only sleep two hours a day. Some marine animals, like porpoises and whales, don’t even sleep with the whole brain. They let one hemisphere sleep while the other stays active, and then swap. They’re always partially awake. Prey animals barely sleep at all — all the better to see the predators sneak up on them. House cats seem to sleep a lot. All. Day. Long. (It’s wonderful. Trust me.)

Humans don’t need eight consecutive hours of sleep. They just need eight total hours every day. The Italian inventor Leonardo Da Vinci practiced polyphasic sleep. That is, he didn’t sleep for eight hours all at once. Instead, he is said to have taken a half hour nap every three or so hours.

Incidentally, he probably didn’t get enough sleep doing this. With this schedule, he would have slept just three hours in a 24-hour cycle.

That’s not enough, says Belenky, the sleep center director. In an experiment of his, he had four different groups of people sleep differing lengths. One group got three hours of “bed time.” Another got five, another seven, and the last got nine hours in bed.

“The people who got nine hours looked amazing,” he says. “The seven and the five’s performance levels went down and then plateaued. The three hour went off the cliff.”

With such fatigue, the brain simply can’t work. With no sleep, 25 percent of capacity to do useful work is lost every 24 hours. If you look at scans of brain activity of a well-rested person when asking them a question, the whole brain lights up and becomes active. That happens even with the simplest question. Not because you need all of the brain, but because the brain is preparing itself for any difficulty of problem.

For instance, if someone is presented with a problem that begins with the number four, the brain evokes everything associated with four. The problem may turn out to be 4+4, which the brain can do with very little energy. But the problem may be 4×58, or 4 divided 7,000, or 4+4+4+4+4+4+4+4+4+4+4+4. The brain doesn’t know, but it readies itself.

With someone deprived of sleep, the brain lights up much less when that four is presented.

“Everything you’re thinking and feeling is just shallower. Everything becomes muted,” says Belenky. “When you’re fatigued the bloom (showing brain activity) shrinks. The glowing area becomes dimmer. You’re slower to get the answer. You may never get the answer. Especially if it entails hanging on, leaping, hanging on, leaping. … You’ll forget what you’re searching for.”

What about the people who say they’re just fine with a few hours of sleep?

“They’re lying,” Belenky says with a laugh. “There are some very few people who actually tolerate (little sleep). But if you looked at them closely you’d probably see they’re hypomanic. They’re edging toward mania. They think they’re performing at their optimum, but they’re not.”

Makes you wonder what Da Vinci would’ve invented with a good night’s sleep.