Sleep Health

There are essentially two states of human life: wakefulness and sleep. The wakeful state ensures efficient learning and work, while good sleep is a prerequisite for maintaining long-term wakefulness. Sleep is another state of brain functional activity.

The brain is not "shut down" during sleep but rather "switches to a different working mode." The occurrence of sleep is the result of the drive of the biological circadian rhythm, which is a complex and precise regulatory process.

The two-process theory model of sleep is the most widely accepted theory in the field of sleep medicine and has been in use ever since. The two-process model posits that sleep is the result of the interaction and regulation between the homeostatic system (Process S – Sleep-wake homeostasis) and the circadian rhythm system (Process C - Circadian rhythm).

I.                    Circadian rhythm system (biological clock) The circadian rhythm is part of the human biological clock and plays a central role in many biological processes in the body (including sleep).

When there is a mismatch between the external environment and the internal biological clock, disruptions may occur. A typical example is the "jet lag" we feel when traveling across multiple time zones. The biological clock not only affects when we sleep and wake up but also influences our daily life rhythm, from metabolism to muscle growth.

The brain's "master clock" (neurons in the suprachiasmatic nucleus of the hypothalamus) sends out signals (such as secreting hormones and regulating body temperature) to set a regular life rhythm and keep the body's functions in an optimal state. The complex human circadian rhythm system also relies on "zeitgebers" to operate.

Zeitgebers are exogenous environmental factors (such as light, temperature, social interaction, exercise, dietary patterns, etc., which are aspects of lifestyle).

They can synchronize the organism's internal clock with the Earth's 24-hour light-dark cycle and 12-month cycle. Improper application can also disrupt the biological clock.

We can also use "zeitgebers" to reset or adjust the biological clock. Key zeitgebers For sleep, natural light is an important zeitgeber.

After the eyes receive natural light, they continuously measure the light intensity and transmit this information to the body's "master clock."

Then, the brain correspondingly controls the body to increase or decrease the secretion of melatonin, thereby affecting our sleep.

Diet is also a zeitgeber that has a significant impact on sleep because the body's ability to metabolize food varies within a 24-hour cycle.

 Research shows that meal times can change the time we go to sleep or wake up. Frequent snacking can disrupt sleep patterns, leading to problems such as weight gain, lethargy, and a slowdown in metabolism.

Some studies even suggest that for certain body systems and functions, meal times may be more important than light. External temperature is also a zeitgeber that can affect the sleep-wake cycle. Exercise is also a zeitgeber. Research has found that exercising in the morning or early afternoon can "advance" the biological clock, allowing us to fall asleep earlier. However, exercising within one or two hours before bedtime may delay the time of falling asleep.

Genetic mechanisms also participate in the operation of the biological clock, and genes can also affect the circadian rhythm system. Studies have shown that everyone has their own unique circadian rhythm phenotype. Roughly speaking, about 40% of the population are "morning larks," 30% are "night owls," and 30% fall in between.

Age, gender, and the external environment (such as light intensity) can affect our individual circadian rhythm phenotypes, and genes are also an important determining factor. The regulatory ability of the biological clock The circadian rhythm has strong resilience and plasticity.

A core concept for improving sleep hygiene is to maintain consistent bedtime and wake-up times according to your circadian rhythm. The influence of the circadian rhythm on the body is the first step in obtaining high-quality sleep. Regular daily routines are the key to keeping the biological clock stable and regular. Going to sleep and waking up regularly, exposing yourself to as much natural light as possible in the morning, and maintaining stable exercise and meal times can all keep the biological clock stable.

 At the same time, we can also formulate work and study plans around the daily peaks and troughs of energy to improve our work efficiency.

II.                  Sleep-wake homeostasis (homeostatic system) In addition to being driven by the circadian rhythm (biological clock), sleep also has a homeostatic mechanism that is controlled by the body's internal biological clock.

Sleep homeostasis is the most important characteristic of the sleep-wake cycle and describes the dynamic balance between sleep and wakefulness. Put simply, sleep homeostasis is mainly closely related to sleep pressure. The sleep pressure of the sleep homeostatic system begins to accumulate after waking up each morning, that is, the "sleep pressure" increases until it's time for you to go to sleep.

As sleep progresses, the sleep pressure is gradually removed. We call this process of sleep pressure accumulation during the day and reset at night the sleep homeostatic system. The key to the normal functioning of the homeostatic system lies in the balance of sleep pressure between day and night. During this period, sleep and wakefulness inhibit and promote each other. If the sleep pressure does not reach the established level before going to sleep, problems such as difficulty falling asleep will occur. Therefore, the principle of the sleep habit of "not going to bed without sleepiness" stems from this.

Currently, it is believed that some metabolic molecules, including adenosine, and intracellular biochemical processes such as protein phosphorylation mediate the increase in sleep pressure during wakefulness. Adenosine is considered an important source of our sleepiness (caffeine is a non-selective adenosine receptor antagonist and a widely used wake-promoting psychostimulant). This organic compound is closely related to sleep pressure and begins to accumulate in the brain after waking up in the morning. As the adenosine level rises, wakefulness declines, and the feeling of drowsiness intensifies. During sleep, the brain starts to clear its own adenosine, thereby effectively resetting the next day's sleep pressure level.

In the sleep homeostatic system, the "seesaw" between the two states of wakefulness and sleep continuously balances itself. If there is insufficient sleep, adenosine cannot be completely cleared, which will exacerbate the negative effects of insomnia. Research has found that after staying awake for 17 consecutive hours, a person's concentration is impaired, and their alertness level is equivalent to a blood alcohol concentration of 0.05%.

After 24 hours of sleep deprivation, the body's state is equivalent to that of being drunk (a blood alcohol concentration of 0.1%), with symptoms such as prolonged reaction time, weakened motor control, reduced judgment, attention, thinking, and memory abilities, and an increased likelihood of negative emotions such as rage and hostility.

More than 30% of fatal car accidents involve problems related to sleep debt (sleep pressure in the homeostatic system). Staying up late and catching up on sleep may help regulate sleep pressure problems. Through clinical scientific means such as professional sleep monitoring and sleep logs, we can accurately depict our unique sleep pressure characteristics.

By cultivating good sleep habits and continuously maintaining the balance of the "seesaw" between the two states of wakefulness and sleep, we will be able to obtain high-quality sleep and a healthy lifestyle and work state. Currently, research on the regulation of sleep homeostasis and sleep rhythm can mainly be divided into two "schools": one starts from the genetic and molecular aspects to study the regulation of sleep homeostasis, and the other starts from the perspective of neural circuits to study the regulation of sleep and wakefulness by different brain regions. The neural mechanism of sleep regulation is very complex.

In recent years, thanks to the advancement of research methods, it has been revealed that the regulation of sleep-wake behavior involves specific types of neurons in multiple different brain regions within the brain.

Sleep and wakefulness are actually the result of the coordinated action of multiple brain regions rather than the regulation of a single specific brain region.

Among them, the basal forebrain is one of the key brain regions, and the thalamus is an important part in charge of sleep and is the earliest brain region discovered to affect sleep in humans. There is a group of γ-aminobutyric acid (GABA)-ergic neurons in the lateral hypothalamus.

The activation of these neurons is associated with sleep behavior, and they have the function of regulating the sleep-wake cycle and are closely related to sleep duration and sleep depth.

Researchers have also pointed out that during the process of accumulating sleepiness after a long period of wakefulness, the glutamatergic neurons in the basal forebrain region of the brain also play an important role.

These neurons not only maintain and promote wakefulness but also lead to an increase in sleepiness by stimulating the release of adenosine, thus facilitating the transition from wakefulness to sleep.

 In addition, in addition to neurons, neural support systems such as astrocytes also play a non-negligible role in the formation of the rhythm of the suprachiasmatic nucleus (SCN).

Astrocytes can promote sleep by releasing adenosine and clean the brain through the lymphatic pathway during sleep. Currently, we do not have effective treatments for most sleep disorders.

One of the main reasons is that our understanding of the brain's mechanism for regulating sleep and wakefulness is not thorough enough.

For example, although adenosine, which regulates the sleep homeostatic system, is known to be related to multiple brain regions, including the basal forebrain, tuberomammillary nucleus, lateral hypothalamic nucleus, and nucleus accumbens, the complete neural circuit in the brain involved in the sleep-promoting effect of adenosine remains unclear. Live well and sleep well!