Sleep seems critical to memory, particularly the
ability to recall recently learned fact and events,
Known as "declarative" memories, these
differ from non-declarative memories, or "how
to" memories -- those have already been shown
to benefit from sleep. However, whether sleep has
an impact on declarative memories has not been known.
This new finding may be particularly important for
people with mentally demanding lifestyles, such as
doctors, medical residents and college students, who
often do not get enough sleep, the researchers say.
"We sought to explore whether sleep has any
impact on memory consolidation," said lead researcher
Dr. Jeffrey M. Ellenbogen, a postdoctoral fellow at
Harvard Medical School's Center for Sleep and Cognition.
"Specifically, the type of memory for facts and
events in time."
The report is published in the July 11 issue of Current
Ellenbogen's team studied 60 people who did not use
prescription drugs and did not have known sleep disorders
or abnormal sleep patterns. Among these, 48 were assigned
to one of four groups: sleep before testing, wake
before testing, sleep before testing with interference,
or wake before testing with interference.
As Ellenbogen explained, "interference is the
concept in memory research that learning some new
piece of information leads to the forgetting of something
else, particularly when that something else is very
In the study, everyone first attempted to memorize
20 paired words. They were tested 12 hours later for
recall by completing a cued-recall task. However,
people in the interference groups were also schooled
in a second list of 20 word-pairs just before testing
-- these were the "distracting" or interfering
words that made remembering the first bunch of word-pairs
In addition, another 12 people were placed on a longer,
24-hour program with either interference and sleep
Ellenbogen's group found that sleep did have a benefit
for declarative memory. People in the non-interference
groups had mean recall that was slightly higher in
the sleep group compared with the wake group. Moreover,
people in the interference group who were able to
sleep still did significantly better on the recall
than did the wake group.
"Sleep had a benefit for the consolidation and
strengthening of memory," Ellenbogen said. "It
actively does so; it's not a passive process. The
brain actively engages memories and leads them to
be strengthened the next day, and it's a long-lasting
benefit. The benefit was even larger than we were
Given these findings, the researchers believe that
sleep is important to building and maintaining memories.
"Sleep is not an inactive state. That's an obsolete
concept," Ellenbogen said. "The brain is
doing lots of things during sleep, including consolidating
memories. So you need to get sleep on a regular basis
in order to maximize memory."
One expert thinks this study shows that sleep is
important in learning.
"Sleep specialists still do not know the overarching
purpose of sleep," said Dr. Robert D. Vorona,
an associate professor in the Division of Sleep Medicine
at Eastern Virginia Medical School, in Norfolk. "However,
we do know that insufficient sleep is associated with
negative alterations in both mood and performance."
A number of studies suggest that sleep plays an important
role in effective memory acquisition, Vorona said.
"This study suggests that parents of students
would do well to recommend that their children both
study hard and obtain sufficient sleep in order to
maximize their academic performance," he added.