The relationship among sleep, learning and memory is still one of the complicated scientific questions that human beings have not yet understood. Studies have shown that the time and quality of sleep have a profound impact on formation of post-learning memory. What role does sleep play in the process of memory formation? Does overnight learning really help with intellectual memory? Can we use scientific means to intervene in memory formation? Regarding these questions, we interviewed Gan Wenbiao, a senior principal investigator of the In stitute of Neurological and Psychiatric Disorders at Shenzhen Bay Laboratory. He and his research team recently published an article in the internationally renowned journal PNAS, revealing the regulation mechanism of post-learning sleep on the formation of new synapses.
(PNAS December 14, 2021 118 (50) e2114856118; https://doi.org/10.1073/pnas.2114856118）
Interviewer: What role does sleep play in the formation of human memory?
Gan Wenbiao: Sleep plays a very important role in formatting the memory, especially in shaping long-term memory. This is mainly reflected in two aspects, one is the consolidation effect of post-learning sleep on memory, and the other is that good sleep helps to learn new tasks after waking up. If we treat the brain as a machine, neurons operate rhythmically, but when the sleep is insufficient and sleep quality is low, the neurons will not be released enough or excessively, then the knowledge learned during the day cannot be integrated and consolidated. And it is also very difficult to recover the state the next day if the brain is not repaired smoothly, thus having an impact on new learning.
Human's sleep stages can be roughly divided into two types, one is slow-wave sleep, including the process from light sleep to deep sleep; the other is REM sleep or dream sleep. The research discovered that these two stages have different effects on the formation of memory.
Interviewer: What is the difference between people's memory formation during sleep and wakefulness?
Gan Wenbiao: From this research, we have observed that some changes have occurred in neuron connections during wakefulness and sleep. We think it has something to do with memory formation. If we regard the brain as a tree, we now know that it will produce new buds in different positions of the trunk during both wakefulness and slow-wave sleep mentioned above, which will have different effects on the shaping of memory. But in general, during slow-wave sleep, more new connections are formed among the neurons, and their roles are more important.
The changes generated during the dreaming sleep are different. One of our previous works found that some newly formed neuron connections are removed during dreaming sleep, but at the same time, some other neuron connections are consolidated and strengthened, which is difficult to be achieved when people are awake. From this point of view, sleep plays a more important role in memory formation.
Interviewer: There are differences in sleep time and memory ability for people at different ages. What is the dynamic relationship among these three?
Gan Wenbiao: The longer sleep time in infancy is mainly for the development and establishment of the nervous system. Of course, it is also helpful for the formation of memory. The adolescence has strong plasticity of their nervous system, relatively better ability to learn new knowledge and good memory. Therefore, we should ensure that the adolescents have enough sleep in their growth stage, so that the learned knowledge can be better consolidated in the brain. Getting to old age, on one hand, the quality of sleep decreases; on the other hand, sleep may play more roles in maintaining the function of the human body and the memory of the past.
There is a phenomenon that some elderly people's short-term memory becomes worse, but they can recall what happened many years ago. We speculate a possibility: the memory formed in the early stage is like the base of a pyramid, which has formed a more stable structure in the long-term shaping. Although there are some losses or deletions in the process, compared with the newly formed memory, the remaining part is more stable and the memory is clearer. However, there is no experimental evidence to prove this directly at present.
Interviewer: Can we currently intervene in the sleep process through scientific means to affect the actual memory ability?
Gan Wenbiao: At present, there are some experiments that specifically intervene in sleep through sound, light, and electricity to observe changes in brain function and behavior, but generally speaking, they are still relatively simple. The human sleep function is the result of a natural process of evolution over hundreds of millions of years. A human brain is a machine that has developed a very good learning and memory function. Using scientific means to intervene in sleep to increase the memory of normal people may be effective to a certain extent, but perhaps will bring some other problems.
It is worth mentioning that we have observed that people with neurodevelopmental diseases such as autism have certain sleep problems without exception. Using scientific methods to adjust the functional mechanism of the neurons during sleep and then to improve development and memory and treat diseases is a very important and meaningful research direction in the future.
Interviewer: Sleep is a part of the human natural biological clock. When staying up late or having jet lag due to cross-country travel, how does the short-term adjustment of the biological clock impact on memory?
Gan Wenbiao: The group experiments on study and sleep found that for the same task, even if the group with insufficient sleep repeated their study one more time, their memory consolidation effect is not as strong as that of the group with adequate sleep. Sometimes we feel that staying up late to study before the exam seems to strengthen our memory in a short time, but it is not the case in the long run. Learning is a lifetime marathon, and sacrificing sleep is not worth the candle. In the long run, even if you spend less time studying to guarantee sleep, the long-term memory effect will be better.
* Dr. Bai Yang, associate researcher of Gan Wenbiao Lab, also contributed to this article.
Further reading: Gan Wenbiao Group has made important research progress in the field of sleep
