Certain memories may rest on a good sleep

When practicing a musical piece, a gymnastics move, or any other activity that depends on effortless, virtually automatic execution, here’s some memory-enhancing advice: If you snooze, you cruise.

That, at least, is the implication of two new studies in which people who practiced a task that demands quick visual processing performed it better on ensuing trials if they were first allowed to get some sleep.

Moreover, one investigation suggests that the initial night of sleep after learning so-called procedural skills proves crucial for memory. The other findings indicate that sleep early in the night, which includes mainly slow-wave electrical activity in the brain, aids procedural recall. Rapid-eye-movement (REM) sleep later in the night strengthens only memories already bolstered by slow-wave sleep, the researchers report.

Both studies appear in the December Nature Neuroscience.

“It’s becoming increasingly clear that sleep is critical for consolidating procedural memory into a usable form,” says neuroscientist Robert Stickgold of Harvard Medical School in Boston.

Stickgold and his colleagues conducted one of the new studies. In an afternoon session, they trained 133 volunteers to perform a visual-discrimination task. Participants viewed, for a fraction of a second, an image including a small set of diagonal bars against a background of horizontal bars. The screen then went blank for a variable period, followed by a distracting pattern for a fraction of a second. Then, the participants reported the orientation of the diagonal bars.

The researchers determined the minimum amount of blank-screen time needed by each volunteer to overcome the distraction and discern the orientation.

Participants who slept on the night after training performed the task better—just as accurately, using less blank-screen time—the following day. They showed further gains over the next 3 days.

In contrast, people deprived of sleep on the night after training didn’t perform the task better the next day. Even when allowed to catch up on their sleep over the next 2 nights, they exhibited little improvement.

“These results suggest that the first night of sleep is crucial for acquisition of the new visual skill,” says neuroscientist Pierre Maquet of University College London in a comment published in the same journal. Maquet has linked brain changes during REM sleep to procedural learning (SN: 7/22/00, p. 55: Available to subscribers at Sleepers yield memorable brain images).

The slow-wave portion of sleep may nevertheless prove essential for procedural-memory formation, according to the second study, directed by Steffen Gais of the Medical University of Lübeck in Germany. Gais and his coworkers trained 21 volunteers on a visual task much like that used by Stickgold.

Performance improved markedly for those who trained for 1 hour in the afternoon and then were tested after sleeping for the first 3 hours of the night, during which the brain shows mainly slow waves. No improvement occurred for those who trained after awakening from the first 3 hours of sleep and were tested after sleeping through the rest of the night.

Volunteers who slept through the entire night after training exhibited a much larger performance boost than those tested after the first 3 hours of sleep. These findings suggest that slow-wave sleep triggers procedural memory formation, whereas REM sleep amplifies that process, Maquet remarks.

However, neither of the new studies establishes sleep as essential to memory consolidation, says Robert Vertes of Florida Atlantic University in Boca Raton. Stickgold’s data also show memory improvement with the mere passage of time, Vertes points out.

Researchers know even less about the effects of sleep deprivation on memories that require conscious effort, Stickgold notes. For instance, staying up all night to cram for a test may work for some students, although they probably forget what they have learned in a few days, he says.