How Snoozing Makes You Smarter

Sleep on It: How Snoozing Makes You Smarter

During slumber, our brain engages in data analysis, from strengthening memories to solving problems


By Robert Stickgold and Jeffrey M. Ellenbogen / Scientific American Mind
August 7, 2008

sleepingdog.jpg
In 1865 Friedrich August Kekulé woke up from a strange dream: he
imagined a snake forming a circle and biting its own tail. Like many
organic chemists of the time, Kekulé had been working feverishly to
describe the true chemical structure of benzene, a problem that
continually eluded understanding. But Kekulé’s dream of a snake
swallowing its tail, so the story goes, helped him to accurately
realize that benzene’s structure formed a ring. This insight paved the
way for a new understanding of organic chemistry and earned Kekulé a
title of nobility in Germany.

Although most of us have not been ennobled, there is something
undeniably familiar about Kekulé’s problem-solving method. Whether
deciding to go to a particular college, accept a challenging job offer
or propose to a future spouse, “sleeping on it” seems to provide the
clarity we need to piece together life’s puzzles. But how does slumber
present us with answers?

The latest research suggests that while we are peacefully asleep our
brain is busily processing the day’s information. It combs through
recently formed memories, stabilizing, copying and filing them, so that
they will be more useful the next day. A night of sleep can make
memories resistant to interference from other information and allow us
to recall them for use more effectively the next morning. And sleep not
only strengthens memories, it also lets the brain sift through newly
formed memories, possibly even identifying what is worth keeping and
selectively maintaining or enhancing these aspects of a memory. When a
picture contains both emotional and unemotional elements, sleep can
save the important emotional parts and let the less relevant background
drift away. It can analyze collections of memories to discover
relations among them or identify the gist of a memory while the
unnecessary details fade—perhaps even helping us find the meaning in
what we have learned.

Not Merely Resting
If you find this news surprising, you are not alone. Until the
mid-1950s, scientists generally assumed that the brain was shut down
while we snoozed. Although German psychologist Hermann Ebbinghaus had
evidence in 1885 that sleep protects simple memories from decay, for
decades researchers attributed the effect to a passive protection
against interference. We forget things, they argued, because all the
new information coming in pushes out the existing memories. But because
there is nothing coming in while we get shut-eye, we simply do not
forget as much.

Then, in 1953, the late physiologists Eugene Aserinsky and Nathaniel
Kleitman of the University of Chicago discovered the rich variations in
brain activity during sleep, and scientists realized they had been
missing something important. Aserinsky and Kleitman found that our
sleep follows a 90-minute cycle, in and out of rapid-eye-movement (REM)
sleep. During REM sleep, our brain waves—the oscillating
electromagnetic signals that result from large-scale brain
activity—look similar to those produced while we are awake. And in
subsequent decades, the late Mircea Steriade of Laval University in
Quebec and other neuroscientists discovered that individual collections
of neurons were independently firing in between these REM phases,
during periods known as slow-wave sleep, when large populations of
brain cells fire synchronously in a steady rhythm of one to four beats
each second. So it became clear that the sleeping brain was not merely
“resting,” either in REM sleep or in slow-wave sleep. Sleep was doing
something different. Something active.

Sleep to Remember
The turning point in our understanding of sleep and memory came in 1994
in a groundbreaking study. Neurobiologists Avi Karni, Dov Sagi and
their colleagues at the Weizmann Institute of Science in Israel showed
that when volunteers got a night of sleep, they improved at a task that
involved rapidly discriminating between objects they saw—but only when
they had had normal amounts of REM sleep. When the subjects were
deprived of REM sleep, the improvement disappeared. The fact that
performance actually rose overnight negated the idea of passive
protection. Something had to be happening within the sleeping brain
that altered the memories formed the day before. But Karni and Sagi
described REM sleep as a permissive state—one that could
allow changes to happen—rather than a necessary one. They proposed that
such unconscious improvements could happen across the day or the night.
What was important, they argued, was that improvements could only occur
during part of the night, during REM.

It was not until
one of us (Stickgold) revisited this question in 2000 that it became
clear that sleep could, in fact, be necessary for this improvement to
occur. Using the same rapid visual discrimination task, we found that
only with more than six hours of sleep did people’s performance improve
over the 24 hours following the learning session. And REM sleep was not
the only important component: slow-wave sleep was equally crucial. In
other words, sleep—in all its phases—does something to improve memory
that being awake does not do.

To understand how that could be so, it helps to review a few memory
basics. When we “encode” information in our brain, the newly minted
memory is actually just beginning a long journey during which it will
be stabilized, enhanced and qualitatively altered, until it bears only
faint resemblance to its original form. Over the first few hours, a
memory can become more stable, resistant to interference from competing
memories. But over longer periods, the brain seems to decide what is
important to remember and what is not—and a detailed memory evolves
into something more like a story.

In 2006 we demonstrated the powerful ability of sleep to stabilize
memories and provided further evidence against the myth that sleep only
passively (and, therefore, transiently) protects memories from
interference. We reasoned that if sleep merely provides a transient
benefit for memory, then memories after sleep should be, once again,
susceptible to interference. We first trained people to memorize pairs
of words in an A-B pattern (for example, “blanket-window”) and then
allowed some of the volunteers to sleep. Later they all learned pairs
in an A-C pattern (“blanket-sneaker”), which were meant to interfere
with their memories of the A-B pairs. As expected, the people who slept
could remember more of the A-B pairs than people who had stayed awake
could. And when we introduced interfering A-C pairs, it was even more
apparent that those who slept had a stronger, more stable memory for
the A-B sets. Sleep changed the memory, making it robust and more
resistant to interference in the coming day.

But sleep’s effects on memory are not limited to stabilization. Over
just the past few years, a number of studies have demonstrated the
sophistication of the memory processing that happens during slumber. In
fact, it appears that as we sleep, the brain might even be dissecting
our memories and retaining only the most salient details. In one study
we created a series of pictures that included either unpleasant or
neutral objects on a neutral background and then had people view the
pictures one after another. Twelve hours later we tested their memories
for the objects and the backgrounds. The results were quite surprising.
Whether the subjects had stayed awake or slept, the accuracy of their
memories dropped by 10 percent for everything. Everything, that is,
except for the memory of the emotionally evocative objects after night
of sleep. Instead of deteriorating, memories for the emotional objects
actually seemed to improve by a few percent overnight, showing about a
15 percent improvement relative to the deteriorating backgrounds. After
a few more nights, one could imagine that little but the emotional
objects would be left. We know this culling happens over time with
real-life events, but now it appears that sleep may play a crucial role
in this evolution of emotional memories.

Precisely how the brain strengthens and enhances memories remains
largely a mystery, although we can make some educated guesses at the
basic mechanism. We know that memories are created by altering the
strengths of connections among hundreds, thousands or perhaps even
millions of neurons, making certain patterns of activity more likely to
recur. These patterns of activity, when reactivated, lead to the recall
of a memory—whether that memory is where we left the car keys or a pair
of words such as “blanket-window.” These changes in synaptic strength
are thought to arise from a molecular process known as long-term
potentiation, which strengthens the connections between pairs of
neurons that fire at the same time. Thus, cells that fire together wire
together, locking the pattern in place for future recall.

During
sleep, the brain reactivates patterns of neural activity that it
performed during the day, thus strengthening the memories by long-term
potentiation. In 1994 neuroscientists Matthew Wilson and Bruce
McNaughton, both then at the University of Arizona, showed this effect
for the first time using rats fitted with implants that monitored their
brain activity. They taught these rats to circle a track to find food,
recording neuronal firing patterns from the rodents’ brains all the
while. Cells in the hippocampus—a brain structure critical for spatial
memory—created a map of the track, with different “place cells” firing
as the rats traversed each region of the track [see “The Matrix in Your Head,”
by James J. Knierim; Scientific American Mind, June/July 2007]. Place
cells correspond so closely to exact physical locations that the
researchers could monitor the rats’ progress around the track simply by
watching which place cells were firing at any given time. And here is
where it gets even more interesting: when Wilson and McNaughton
continued to record from these place cells as the rats slept, they saw
the cells continuing to fire in the same order—as if the rats were
“practicing” running around the track in their sleep.

As this unconscious rehearsing strengthens memory, something more
complex is happening as well—the brain may be selectively rehearsing
the more difficult aspects of a task. For instance, Matthew P. Walker’s
work at Harvard Medical School in 2005 demonstrated that when subjects
learned to type complicated sequences such as 4-1-3-2-4 on a keyboard
(much like learning a new piano score), sleeping between practice
sessions led to faster and more coordinated finger movements. But on
more careful examination, he found that people were not simply getting
faster overall on this typing task. Instead each subject was getting
faster on those particular keystroke sequences at which he or she was
worst.

The brain accomplishes this improvement, at least in part, by moving
the memory for these sequences overnight. Using functional magnetic
resonance imaging, Walker showed that his subjects used different brain
regions to control their typing after they had slept. The next day
typing elicited more activity in the right primary motor cortex, medial
prefrontal lobe, hippocampus and left cerebellum—places that would
support faster and more precise key-press movements—and less activity
in the parietal cortices, left insula, temporal pole and frontopolar
region, areas whose suppression indicates reduced conscious and
emotional effort. The entire memory got strengthened, but especially
the parts that needed it most, and sleep was doing this work by using
different parts of the brain than were used while learning the task.

Solutions in the Dark
These effects of sleep on memory are impressive. Adding to the
excitement, recent discoveries show that sleep also facilitates the
active analysis of new memories, enabling the brain to solve problems
and infer new information. In 2007 one of us (Ellenbogen) showed that
the brain learns while we are asleep. The study used a transitive
inference task; for example, if Bill is older than Carol and Carol is
older than Pierre, the laws of transitivity make it clear that Bill is
older than Pierre. Making this inference requires stitching those two
fragments of information together. People and animals tend to make
these transitive inferences without much conscious thought, and the
ability to do so serves as an enormously helpful cognitive skill: we
discover new information (Bill is older than Pierre) without ever
learning it directly.

The inference seems obvious in Bill and Pierre’s case, but in the
experiment, we used abstract colored shapes that have no intuitive
relation to one another, making the task more challenging. We taught
people so-called premise pairs—they learned to choose, for example, the
orange oval over the turquoise one, turquoise over green, green over
paisley, and so on. The premise pairs imply a hierarchy—if orange is a
better choice than turquoise and turquoise is preferred to green, then
orange should win over green. But when we tested the subjects on these
novel pairings 20 minutes after they learned the premise pairs, they
had not yet discovered these hidden relations. They chose green just as
often as they chose orange, performing no better than chance.

When
we tested subjects 12 hours later on the same day, however, they made
the correct choice 70 percent of the time. Simply allowing time to pass
enabled the brain to calculate and learn these transitive inferences.
And people who slept during the 12 hours performed significantly
better, linking the most distant pairs (such as orange versus paisley)
with 90 percent accuracy. So it seems the brain needs time after we
learn information to process it, connecting the dots, so to speak—and
sleep provides the maximum benefit.

In a 2004 study Ullrich Wagner and others in Jan Born’s laboratory
at the University of Lübeck in Germany elegantly demonstrated just how
powerful sleep’s processing of memories can be. They taught subjects
how to solve a particular type of mathematical problem by using a long
and tedious procedure and had them practice it about 100 times. The
subjects were then sent away and told to come back 12 hours later, when
they were instructed to try it another 200 times.

What the researchers had not told their subjects was that there is a
much simpler way to solve these problems. The researchers could tell if
and when subjects gained insight into this shortcut, because their
speed would suddenly increase. Many of the subjects did, in fact,
discover the trick during the second session. But when they got a
night’s worth of sleep between the two sessions, they were more than
two and a half times more likely to figure it out—59 percent of the
subjects who slept found the trick, compared with only 23 percent of
those who stayed awake between the sessions. Somehow the sleeping brain
was solving this problem, without even knowing that there was a problem
to solve.

The Need to Sleep
As exciting findings such as these come in more and more rapidly, we
are becoming sure of one thing: while we sleep, our brain is anything
but inactive. It is now clear that sleep can consolidate memories by
enhancing and stabilizing them and by finding patterns within studied
material even when we do not know that patterns might be there. It is
also obvious that skimping on sleep stymies these crucial cognitive
processes: some aspects of memory consolidation only happen with more
than six hours of sleep. Miss a night, and the day’s memories might be
compromised—an unsettling thought in our fast-paced, sleep-deprived
society.

But the question remains: Why did we evolve in such a way that
certain cognitive functions happen only while we are asleep? Would it
not seem to make more sense to have these operations going on in the
daytime? Part of the answer might be that the evolutionary pressures
for sleep existed long before higher cognition—functions such as immune
system regulation and efficient energy usage (for instance, hunt in the
day and rest at night) are only two of the many reasons it makes sense
to sleep on a planet that alternates between light and darkness. And
because we already had evolutionary pressure to sleep, the theory goes,
the brain evolved to use that time wisely by processing information
from the previous day: acquire by day; process by night.

Or it might have been the other way around. Memory processing seems
to be the only function of sleep that actually requires an organism to
truly sleep—that is, to become unaware of its surroundings and stop
processing incoming sensory signals. This unconscious cognition appears
to demand the same brain resources used for processing incoming signals
when awake. The brain, therefore, might have to shut off external
inputs to get this job done. In contrast, although other functions such
as immune system regulation might be more readily performed when an
organism is inactive, there does not seem to be any reason why the
organism would need to lose awareness. Thus, it may be these other
functions that have been added to take advantage of the sleep that had
already evolved for memory.

Many other questions remain about
our nighttime cognition, however it might have evolved. Exactly how
does the brain accomplish this memory processing? What are the chemical
or molecular activities that account for these effects? These questions
raise a larger issue about memory in general: What makes the brain
remember certain pieces of information and forget others? We think the
lesson here is that understanding sleep will ultimately help us to
better understand memory.

The task might seem daunting, but these puzzles are the kind on
which scientists thrive—and they can be answered. First, we will have
to design and carry out more and more experiments, slowly teasing out
answers. But equally important, we are going to have to sleep on it.

Note: This article was originally published with the title, "Quiet! Sleeping Brain at Work."

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