Study: Final Sleep Hours Are Critical

The wannabe super-achiever who spends too much time getting ready for the big day at the expense of getting enough sleep is probably doing precisely the wrong thing.

Researchers at Harvard University have found new evidence that the brain continually reorganizes itself while the host is snoozing away. A nap in the afternoon probably boosts performance, but it's no substitute for getting a full night's sleep because those last couple of hours of slumber are when the brain tells itself to pay attention to what's going on.

It is during that critical phase that the brain sends "funny looking wave forms" that tell some brain cells to beef up their connections with other neurons, says Mathew Walker of the Harvard Medical School's department of psychiatry. Walker, lead author of a report published in the July 3 issue of the journal Neuron, says those wave forms apparently work as "powerful triggers" that tell our brain something we've been working on is important, thus improving our performance the next day by committing a learned skilled to the memory banks.

Honing Skills in Your Sleep

That electrical activity, called "sleep spindles," can be measured, and the research shows that spindles abound toward the end of the sleep period.

"It just so happens that when you track the amount of these spindles that the human brain has during the regular eight-hour period of sleep, they seem to ramp up and have very high intensities in the last two hours of the night," Walker says. "It's specifically those two hours that we have found to be most important."

So the person who hops out of bed after six hours sleep and goes forth to use newly learned skills to slay a dragon has managed to shut down that critical phase of mental activity just as it was getting started. His new skills likely won't be nearly as potent as they might have been.

Significantly, this apparently applies to a wide range of human endeavors, from playing a musical instrument to hitting a golf ball to pounding on a keyboard. All of those depend on something scientists call the "procedural memory system," and it involves learning all types of skills.

For part of the research, Walker and his colleagues took 62 right-handed persons and asked them to type a sequence of numbers (4-1-3-2-4) with their left hand as quickly and as accurately as possible. On average the performance of the participants improved about 60 percent just by repeating the task over and over.

Nap Minimum: One Hour

The participants were then divided into subgroups, and one group was tested again after staying awake for 12 hours. They showed no improvement. But when tested after a full night's sleep, they improved by 19 percent.

Another group improved by 20.5 percent after a night's sleep, but gained only 2 percent after staying awake for 12 hours. After a full night's sleep, their performance jumped again by nearly 20 percent.

Other research showed that the improvement in performance was directly related to the completion of a full eight hours of sleep. The last two hours, Walker says, turned out to be the most important.

In a related project, other Harvard researchers found that a midday nap does much to recharge the system by allowing the brain to consolidate the memories of habits, actions and skills learned during the day, according to Robert Stickgold of the department of psychiatry. A nap, he says, gives the brain a chance to avoid "burnout" and reorganize itself, perhaps bringing new circuits in to relieve some that had grown weary.

That short of a snooze, however, is no substitute for a full night's sleep, the researchers say.

And to be much help at all, the nap needs to be about an hour long. Try explaining that one to the boss.

This is still a relatively new field, and much remains to be learned about what the brain is up to while we think it's just sleeping, but what is emerging loud and clear is the fact that sleep is far more critical than most of us might think.

Sleep = Work

We may think our brain is "dormant" while we're asleep, Walker says, but it's actually firing off commands and doing all sorts of things to restore our biological systems to good working order.

"You spend about a third of your life asleep, and I don't think mother nature is stupid enough to keep you asleep for a third of your life to do just one thing and one thing only," like rest your weary bones, he says.

"You don't spend two thirds of your life awake just doing one thing. You do many things while you are awake."

Researchers are finding, he says, that parts of the brain are as active when we are asleep as when we are awake."

And the new research suggests that if we cut back even a little on our sleep, we're reducing the brain's ability to commit what we have learned to memory, whether it's typing, designing a rocket engine, or learning a new dance step.

It's enough to give you nightmares.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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