Sleep Cycle and Digital Treatment

Hi, welcome to the lecture I’m Chang-Wei Wu. Today I am going to introduce the basic ideas about sleep and its relation with consciousness. Here is my favorite poem about sleep, “Sleep is a temporary death. Death is a permanent sleep. Every day is your birthday ‘cause you’re reborn again” This poem is actually to ask people to cherish their life and seize the moment during wakefulness. Meanwhile, it also describes the external appearance of a person in sleep. Without any body movement and no response to the external stimuli, the sleeping condition seems like the loss of consciousness, which makes sleep look like a temporary death.

However, right now we know sleep is not a death-like condition, but a transient change of consciousness levels in the brain.

This chart describes the levels of consciousness. You can find out every night we experience a reduction of consciousness levels in sleep, and after a long night, you get rid of the “temporary death” and recover back to fully conscious state in the morning. Why does sleep, the vulnerable condition, happen everyday and occupy almost 1/3 of our life in most mammals and birds? This is still a mystery and we are still looking for the answers. That is why Dr.

Allan Rechtschaffen said: “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.”

Therefore, to understand more about sleep, our first step is to know what we know right now about sleep itself.

So, the first question is: why do we sleep? Dr.

Mignot in 2008 briefly introduced 3 current physiological theories in sleep: First, mammal behavior peaks at specific times in terms of performance and food availability. It is a better strategy in evolution to reduce energy consumption at other times, ensuring survival when food is scarce. Second, learning during wakefulness leads to long-term potentiations and strengthening of glutamatergic synapses in the brain. In sleep, a proportional synaptic pruning process occurs, leaving only the most robust connections and reducing energetic requirements for the maintenance of crucial learned circuits. Third, Protein synthesis has been shown to increase during sleep, suggesting sleep function is to restore macromolecules and replenish transmitter vesicles that have been used by extended wakefulness.

These 3 theories are not exclusive to each other, and scientists are still working on more objective measurements to support the necessity of sleep functionality. So, now we know that sleep comes from our brain. It leads to the question that, what kind of measurement was performed to realize more about sleep?

The current gold standard is what we call sleep polysomnography, or PSG, to measure breath, blood oxygenation, brain waves and muscle tones over time. PSG system originated from the EEG findings of Dr. Eugene Aserinsky. In 1953, he found a certain period of time in sleep that participants moved their eyes swiftly. This was the first discovery of rapid-eye-movement sleep, or REM sleep. His REM sleep findings attracted the entire world because about 74% of participants waked from REM sleep reported visually vivid and bizarre dreams, but the brain wave pattern was almost similar as that in wakefulness with physical atonia. So REM sleep is also known as the paradoxical sleep and dreaming period.

Except REM sleep, Dr. William Dement continued Aserinsky’s work on EEG brain waves, segregated different brain waves into 4 Non-REM sleep stages. Incorporating with Wake and REM-sleep, the 6 stages became the foundation of current sleep scoring. Among the sleep stages, we spent about half of time is Stage 2 and approximately 25% in REM sleep. According to the newest sleep scoring criteria, Stage 3 and 4 are combined as the slow-wave sleep, or just simply stage 3.

If we conduct sleep scoring over the entire night, we would observe the NREM and REM sleep distribute in a interleaved manner, forming the so-called ‘sleep cycles’. The first cycle takes about 90 minutes in our first night. In the first part of the night. According to this figure, slow-wave sleep dominates in the first half of a night, and REM sleep dominates in the last half of a night. So, SWS and REM sleep facilitate distinct cognitive functions in the night time. For example, NREM sleep facilitates declarative memories, whereas REM sleep boosts the procedural memory and emotions.

So, how did scientists conduct this study? For example, Dr. Matthew Walker recruited two groups of participants and asked them to learn a piano motor sequence at different timing. One group learned in the morning, and the other learned in the evening. After 12 hours of purely waking and 12-hour periods with nocturnal sleep, the participants were asked to perform the piano sequence again in a morning or in a night. Dr. Walker showed that no matter which group it is, after a night of sleep, it was able to trigger significant improvements on speed and accuracy of piano finger tapping task, while equivalent periods of wakefulness provided no significant benefits. Furthermore, overnight learning improvements correlate with the amount of stage 2 NREM sleep.

This means that sleep helps us to consolidate newly-learned memory. So, try not to burn the midnight oil all night if you are having an examination on next day.

Besides, another benefit from sleep is to regulate our emotion. Dr. Walker also conducted declarative memory experiments without having his participants falling into sleep. He found 40% of memory loss under 36-hour sleep deprivation. When separating the memory contents into positive emotion, negative emotion and neutral, they found 59% of memory reduction in positive emotion contents. This indicates that without sleep, your happy thoughts will not be stored into your long-term memory, but negative feelings will remain. How scary is it?