March 7, 2000

For Better Learning, Researchers Endorse 'Sleep on It' Adage

By SANDRA BLAKESLEE

Scientists have discovered a new link between sleep and memory that seriously challenges the wisdom of skimping on time spent sleeping. 

Experiments show that when people learn a new skill, their performance does not improve until after they have had more than six and preferably eight hours of sleep. Without adequate sleep, researchers say, skills and even new factual information may not get properly encoded into the brain's memory circuits. Moreover, a person's intelligence may be less important than a good night's sleep in forming many kinds of memories. 

Looks like Mom was right: if you don't get enough sleep, you won't do as well.

The experiments, described in the March issue of The Journal of Cognitive Neuroscience, point to a new hypothesis for memory formation involving the interaction between two stages of sleep, one that occurs at the beginning of the night and one that occurs early in the morning. 

During both periods the brain undergoes physical and chemical changes whose interaction may be what strengthens memory traces. 

The research was carried out by Dr. Robert Stickgold, an assistant professor of psychiatry at Harvard Medical School, and his colleagues in the Laboratory of Neurophysiology at the Massachusetts Mental Health Center in Boston. 

"This is the first study to show that humans have a sleep window for learning and memory," said Dr. Carlyle Smith, a professor of psychology at Trent University in Peterborough, Ontario, and a sleep study expert. "It means that people who cut their sleep short for the last couple of hours each night generally won't do as well as those who get a full night's sleep." 

Dr. Chris Gillin, a professor of psychiatry at the University of California in San Diego, called the study intriguing and described it as "the most believable data ever collected that a specific memory function is associated with sleep." 

The research also has important implications for institutions -- like universities, medical schools and the military -- that often train people amid long bouts of sleep deprivation. That is, more sleep might improve students' performance. 

To explore the link between learning and sleep, the researchers trained Harvard undergraduates to spot visual targets on a computer screen and to press a button as soon as they were certain they had seen one. 

At first, it might take 400 milliseconds for a target to reach the students' conscious awareness, Dr. Stickgold said. 

But with an hour or so of practice they could reliably see the targets much faster. 

For example, at the end of training a student might accurately press a button in 75 milliseconds. 

Students who are trained to do this task and are retested from 3 to 12 hours later on the same day show absolutely no improvement in speed beyond their best time at the end of training, Dr. Stickgold said. 

And the students who sleep six hours or less after training also show no improvement when they perform the same task the next day. 

Only those who sleep more than six hours seem to improve. 

For example, someone whose best time was 75 milliseconds at the end of training might, after a good night's sleep, reliably perform the task in 62 milliseconds. 

This improvement in speed and accuracy is somehow consolidated during sleep, Dr. Stickgold said. 

But it is not just any kind of sleep that matters. 

The students who improved the most slept for eight hours, during which they got solid bouts of two kinds of sleep as measured in a sleep laboratory. 

The first two hours of the night were spent in deep, slow-wave sleep. The last two hours were spent in rapid eye movement sleep when vivid dreams occur. 

People needed both kinds of sleep to do better on what they had learned the day before, he said. Moreover, the good night's sleep continued to pay dividends. Well-rested students tested two days to a week after training could do the visual task even faster. 

Dr. Stickgold has a hypothesis about the need for the two types of sleep. 

During the first two hours of slow-wave sleep, Dr. Stickgold said, certain brain chemicals plummet and information flows out of a memory region called the hippocampus and into the cortex. 

During the next four hours, he theorizes, the brain then engages in a kind of internal dialogue that distributes this new information into the appropriate networks and categories. 

A slow process of protein synthesis begins to strengthen connections between nerve cells that have newly acquired information. 

"It's as if you have to wait for the dough to rise," Dr. Stickgold said. 

In the last two hours of sleep, brain chemistry and activity again change drastically as the cortex goes into an active dreaming state. The hippocampus is shut off from the cortex as the brain literally re-enacts the training and solidifies the newly made connections throughout its memory banks. 

"As I tell students, this is one of those 'your-mother-was-right' studies," Dr. Stickgold said. 

"If you don't get good sleep and enough sleep after you learn new stuff, you won't integrate it effectively into your memories," he said. "The six-hour cutoff really scares students." 

Many college students suffer from a kind of sleep bulimia in which they binge and purge sleep time, Dr. Stickgold said. 

They get by on three to five hours a night during the week and binge on weekends thinking that they will be fine. 

But much of the information learned during a sleep-deprived week will not be well-integrated into memory circuits. 

After a few days, facts memorized during an "all nighter" tend to fade away, Dr. Stickgold said. 

It seems that REM sleep is important for integrating all kinds of information into the brain over the long haul, he added. 

"The study also challenges expectations and prejudices about what makes a smart student," Dr. Stickgold said. "How well Harvard undergrads do the next day on a retest does not depend on what prep school they went to, their SAT scores or how hard they tried. Rather, it mostly depends on how well they slept." 

While the visual task was set up for the laboratory, it has implications for things people do in the real world, Dr. Stickgold said. 

The study shows that learning a task involving skilled movements and memories for procedures -- things like mastering a sport or playing a musical instrument -- will definitely be helped by a good night's sleep. 

But so will other types of learning, including some kinds of rote memorization, he said. To learn and remember new things, many parts of the brain are called into play. 

Students admit that facts stuffed into their heads, after the proverbial all-nighter, tend to fade away in two to three days. 

Long-term memory and learning involve putting all the facts and procedures together in novel ways, Dr. Stickgold said. 

"In any kind of learning," he said, "it's a great idea to sleep on it."