My son Gil recently took a final exam in nursing school after cramming the whole night. He did well, thank you. But when he told me about this feat of studiousness, I was wondering, how much is he going to retain one day after the test? I seem to recall from my college days that taking a test after an all-nighter was actually fraught with a lot of fuzzy thinking and faulty memory. Have you ever taken a test, and later asked yourself “how could I make such a stupid mistake?” or “how could I forget that?”
Perchance to sleep
Sleep is an essential activity in our daily life. Since the beginning of time, immemorial sleep was associated with healing and healthfulness. Lack of sleep was associated with hallucinations and psychic torment. Just ask Lady Macbeth, walking all night, tormented by visions of blood and murder, longing for some blissful rest. Or the captives of state organs, subjected to sleep deprivation in order to wring some information or an admission out of them.
So why is sleep necessary? It has been the subject of lively debate among psychologists and neuroscientists for many years. Still, there is no answer. Some scientists even claimed that it actually is not necessary. One of my colleagues claimed that he never went to sleep for longer than an hour or two a night. But every time I visited him in his office at the university, I found him dozing at his desk. In fact, if sleep were not necessary, it would have been a colossal evolutionary waste; evolution just doesn’t work that way.
Indeed, recent experiments convincingly demonstrated at least one function.
Human sleep is divided into REM, or rapid eye movement, and NREM, or non-REM, sleep. NREM is further divided into 4 stages, each characterized by typical electrical oscillations seen on an encephalogram or EEG.
- REM sleep is characterized by theta waves, having oscillations of 4-8 Hz (Herz is a measure of frequency).
- Stage 1 is transitional between wakefulness and sleep and is, therefore, characterized by a mixture of wave frequencies.
- Stage 2 is typified by spindle-shaped waves of 12-14 Hz.
- Stage 3 and 4 are characterized by delta waves of 1-4 Hz.
- Interwoven in the spindle waves and the delta waves patterns, one can discern another oscillatory pattern: extremely slow, less than 1 Hz, wave patterns called cortical slow oscillations, or slow-wave sleep.
Marshall et al. did an ingenious experiment (Marshall et al. Nature, vol.444, pp.610-613, 2006). They wanted to know if any of these sleep phases were associated with consolidation of memory. So, they let 13 volunteers memorize 46 word-pairs before going to sleep. Electrodes were then attached to their scalp, and fluctuating electrical potentials were then applied to half the group to induce cortical slow oscillations. The other half received sham stimulation.
The next morning the two groups were tested on the word-pairs they had memorized the night before. Results were quite remarkable. The group that had received the slow-wave stimulation performed better than the sham stimulation group. And they also performed better than they, themselves, had performed the night before. The effect was absolutely specific to slow-wave stimulation. No effect was shown when the subjects were stimulated with theta waves frequency of REM sleep. Timing of the stimulation was also important: The first 45 minutes of sleep enhanced recall, the last 45 minutes had no effect.
What did we learn?
- First, we now know for certain that sleep is important in consolidating memory.
- We also know that only certain brain waves (slow oscillating) are associated with memory consolidation.
- Finally, we know that if we want to do well on the test the next day, we’ve got to get a minimum of an hour’s sleep. This is when memories are getting embedded in our brain.
Can we enhance memory by artificial means?
Commercial claims notwithstanding (“learn a language while you sleep”), sleep-learning is a pipe dream; it doesn’t work. If you are willing to go to sleep with electrodes attached to your skull, you might be able to recall your Spanish lesson from the day before somewhat better. But those pesky electrodes will probably wake you up throughout the night, and you’ll feel like (fill in your favorite expression) the next day.
Can this experiment point the way to a “memory drug”?
Unfortunately, not. Drugs work on chemicals in the body, not on electrical waves. We don’t know from this experiment which neurotransmitters are associated with the slow-wave oscillations. Acetylcholine, a known neurotransmitter associated with memory retention, has not been tested. Astonishing. They could have nailed it. Maybe they wanted to get another paper out of it. We’ll just have to wait.
Take home lesson
Gil, next time, get some sleep; even an hour will ensure a better grade.