Can Sleep Enhance Memory Power?

By guest author Dr. Steven Chang

When I reviewed for United States Medical Licensing Exam (USMLE) last year, I enrolled at Kaplan Review Center. Aside from the fact that they have good books and useful resources, they also have good teachers who give valuable tips about exam preparation. I could never forget my behavioral science teacher who pointed out, that the secret to maximize memory inputs is simply to make sure that I should not deprive myself of sleep, despite the piles of books and review materials that I still have to read. I took that tip seriously from day 1 of my review to the day of my exam. When the result of the test came out, I could not believe what I saw. I got 95%! I never thought that this could be possible since I’ve never been a top student in my medical class after all.

But thinking about it, maybe sleep has indeed contributed to the outcome of my exam. This I can say, because back in my medical years while in school, burning the midnight candle was my thing. I wouldn’t sleep until I’m done reading, causing me to be sleep-deprived most of the time.

Sleep is an unconscious state in which the human brain is more receptive to internal than external stimulus (e.g. visual, auditory, environmental stimuli). A normal sleep is divided into non-rapid eye movement (NREM) and rapid eye movement (REM). NREM sleep is characterized by absence of eye movement and thoughtlike mental activity, slowing of the EEG rhythms, and high muscle tone. Simply stated, the brain is inactive while the body is active. NREM is further divided into 4 stages: stage 1, stage 2, stage 3, and stage 4. As NREM stages progress, stronger external stimuli are needed to bring about awakening. Thus, when the person is at stages 3 or 4 of his sleep, he is very hard to arouse. These stages, however, vanish in elderly.

The REM sleep, on the other hand, is when the brain is active and the body is inactive. It is characterized by saccadic eye movement, generalized muscle paralysis, aroused EEG patterns (bursts of sawtooth waves), and sexual arousal. When the person is dreaming, he is said to be in REM sleep. This stage is easiest to arouse.

During the first 4 hours of sleep, you can be anywhere in the stage of REM or NREM. But as the time of sleep progresses, you will be most of the time in REM or stage 1 of NREM and these are the sleep stages where you will usually wake up from.  When you awaken and remember that you were dreaming prior to waking up, then you woke up from REM sleep. But if you awaken and remember nothing, then you are said to wake up from a NREM (stage 1) sleep.

The effect of sleep in memory performance can be demonstrated by a study in which college students were trained to identify similar-sounding words produced by a speech machine. Participants performed well immediately after the training that morning, but performance declined when retested later in the day. However, when they were retested the next day, after a full night sleep, they performed just as well as their original levels.

From the innumerable studies that were conducted about sleep and its effect on memory, researches have come to identify 3 stages of memory processing. The first stage is stabilization that seems to take around 6 hours. During this period, memory is particularly labile and likely to be lost. This stage is analogized by researchers with that of saving a word document, wherein you press “Save” and the computer files the document to the hard drive. It only takes seconds for the computer though, while 6 hours for the human brain.

The second stage of memory processing is consolidation (encoding) which occur during sleep, most especially during REM sleep and stages 3 and 4 of NREM sleep. At this time,  new memories that are vulnerable to disruption are encoded and undergo series of changes (like neural growth and  rearrangement, protein synthesis, and glutamate release) making the memory representations more stable. This stage is tantamount to someone cleaning up your saved word document, reorganizing it and tightening it up for faster recovery when needed.

The third and final stage is recall, when information is recovered from your memory. This is oftentimes referred to as reactivation wherein we literally recall the information we have saved.  Recall is necessary in order for stable memory not to shift back to labile state. Without recall, you forgot that you have saved such information before, simply because you failed to retrieve it every now and then. I guess this is the rationale why practice tests are given after every chapter in school books.

Just like computers which need time to process data, human brains too, need time to process memory. We can not underrate the fact that vital information which we dire need to commit into memory must be consolidated and encoded properly. And since consolidation happens during sleep, we must therefore not deprive ourselves of sleep. For all we know, that maybe the best way to maximize our brain’s memory potential.

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