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The Effect of Sleep on Memory
Jessie Hamlett
PSY150-61Y
12 December 2016
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Introduction
It has previously been concluded that sleep plays a vital role in memory consolidation in
the brain. Sleep has been determined to increase procedural and declarative memory in adults.
REM sleep is important in the process of consolidating memories and a lack of REM and slow
wave sleep has been linked to a decrease in declarative memory and a decreased ability to evoke
visual memory. Spencer et al discovered similar results in younger and older adults in initial
procedural memory. But, the older subject’s performance did not improve after sleep this
proposes the idea that as a person ages their sleep dependent memory declines. There is a
noticeable decrease in slow wave sleep throughout adolescence, declining by over 60% between
the ages of 10 and 20 years of age, following an increase shortly before puberty. In order to
comprehend the significance of interrupted sleep in adolescence in is necessary to understand a
normal sleep. This is critical because many adolescents fail to receive the necessary quantity of
sleep and interrupted sleep is a crucial piece in the majority of juvenile disorders. Backhaus et al.
studied 27 children between the ages of 9 and 12, with a mean age of 10.1 years, testing memory
using a variety of test. They discovered that declarative memory was improved significantly
following sleep and with delayed post-learning sleep. However, this study had no control, so a
new study will attempt to address previous restrictions by exercising control and gaging the
effect that sleep has on an adolescent’s auditory declarative memory. They predict that if
adolescents obtain an adequate amount of undisturbed sleep, then their auditory declarative
memory will improve.
Methods
The study recruited 40 students from a public middle school ranging from 10 to 14 years
old, with 20 male volunteers and 20 female volunteers. The study excluded students with
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academic failure, enhanced performance, or sleep problems. The subjects were asked to
participate in two tests, lasting approximately 15 minutes in duration, for a school project. Then,
the subjects who agreed were blocked off by gender and randomized into either sleep conditions
or no sleep conditions. Separate tables for random selection were used for each group to
guarantee a random and stable design.
The students were tested in a quiet room, free of distractions, in their homes over breaks
from school or over weekends. The students were all tested on declarative memory using a
paired-associate test. In this test the subjects had to repeat pairs of words, some related and some
unrelated, the subject repeated back each pair aloud to guarantee comprehension. Ten pairs of
words were repeated three times in instant sequence. The subjects in the sleep group were taught
the pairs at 9PM ( + 30 minutes), slept for the night, and were tested 12 hours after learning the
pairs. However, the group with no sleep conditions learned the words at 9AM ( + 30 minutes)
and were tested 12 hours later without any sleep. Prior to learning the pairs and testing on the
pairs, all of the subjects were given a letter-number test, a control working memory task. This
test was directed in order to have control in the event of any circadian peculiarities and to control
for concentration and encryption. This task is a subtest of WAIS-III (Wechsler Adult Intelligence
Scale), as well as WMS-III, and is the most widely used test for memory and intelligence. In this
study there were two randomized versions of this test. The memory test’s results were converted
into z-scores in order to test outliers, then any z-scores of + 2.57(1% of the normal distribution)
were excluded. The groups were compared using t-tests, following equal variance testing. Paired
t-tests were used in order to compare within subjects. Prior to the testing all of the students were
told to eat their regular meals an hour before learning and testing and acquire a good night’s
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sleep. Every student reported a typical night’s sleep with good or very good sleep quality prior to
testing.
Results
The average age of the sleep group, 12.9, was 0.5 years older than the no sleep group,
12.4, this is not statistically significant due to a relatively high p-value of 0.14. The differences
between sexes were also not statistically significant for either task. In this study there were three
outliers, all of which were removed so the results would not be skewed. One male in the sleep
group had a z-score exceeding 2.57 and two females in the no sleep group scored a z-score of
less than -2.57. After accounting for outliers, Levene’s test displayed equality on all variances
used for evaluation. Initially the sleep group averaged 6.58 correct responses on the letternumber task, which was 0.52 correct responses higher than the no sleep group who had a mean
of 6.06 correct responses. The probability of this occurring by chance alone is 0.14, which is not
statistically significant. When the subjects took the test a second time the scores were 6.26 and
6.33, respectively. Again, this is not statistically significant, due to a relatively high p-value of
0.88. The differences in the performances of both groups between the first and second attempts
were also insignificant. In the sleep group the p-value 0.32 and in the no sleep group the p-value
was 0.45. However, based on the paired-associate test, there was an increase of 20.6% in the
number of correct answers in the sleep group when compared to the no sleep group. The
probability of this increase occurring by chance alone is less than 0.029 which is statistically
significant due to a p-value of less than 0.05. This significance is increased even more when the
researchers include the outliers, the three subjects previously excluded due to z-scores surpassing
or falling short of the margin of + 2.57. The sleep group had a mean of 7.5 correct responses,
compared to 5.9 by the no sleep group. This is a 32.7% increase with a p-value of less than
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0.009, which is statistically significant, because it surpasses the widely accepted margin of
p=0.05.
Assessment
This was a well done and thorough study about the effect of sleep on memory in
adolescents. Nearly all of the confounding variables were addressed such as gender, academic
performance, and sleep habits. The study met the requirements of an experiment: randomization,
control, and repeatability. All of the subjects were blocked by sex and then randomized into
groups and the tests were also in random order. They identified outliers in order to avoid the data
becoming skewed and unreliable, and they used Levene’s test to show the equality of variances
as much as possible for all areas of comparison. However, there still could have been
confounding variables, especially in regards to the subject’s sleep. Although all of the students
reported having had a good night’s sleep it is impossible to really measure and compare the
quality of their sleep, and the duration was not measured either. This could potentially throw off
the results, based on each student’s idea of a good sleep. Also, only 40 students were tested at
one public middle school in this study, which is most likely due to convenience bias, and the
sample was relatively small and did not representative of the population. Therefore the
researchers are not able to generalize to a larger population with their results. On the whole this
was a good study about sleep’s effects on memory and is a starting point for further and more in
depth analysis of all of the effects in various areas of an adolescent’s life.
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References
Potkin KT, Bunney WE Jr (2012) Sleep Improves Memory: The Effect of Sleep on Long Term
Memory in Early Adolescence. PLoS ONE 7(8): e42191.
doi:10.1371/journal.pone.0042191