How the sleep-wake transition occurs

The wake-sleep transition is a surprising phenomenon about which we do not know everything. Although it is a vital and natural process that occurs with a circadian periodicity, the truth is that many people suffer alterations in this regard.

From the well-known insomnia to less common disorders, such as narcolepsy, Knowing how these transitions work can help us design appropriate interventions to improve the quality of life of those who suffer from them.

Even in healthy individuals, going from wakefulness to sleep and vice versa can be complicated. Many nights we find it difficult to fall asleep and many mornings we have difficulty clearing our heads and regaining activity.

These changes from one state to another are really remarkable, and our state of consciousness and our behavior are really different when we are asleep and when we are awake, but so is the functioning of our brain.

So what do we really know about it?

During sleep, global slow waves occur.

This is how the wake-sleep transition occurs

Neuronal dynamics change noticeably from wakefulness to sleep. While we are awake, we can observe desynchronized activity using an electroencephalogram; instead, When we fall asleep, we observe globally synchronized slow wave activity. However, this transformation is neither drastic nor instantaneous.

Recent research has found how this transition occurs. The findings show that Local slow waves appear already during wakefulness, and that slow sleep waves are rarely global. Its appearance seems to be related to the decrease in arousal. Thus, when cholinergic neuromodulation decreases, local slow waves appear. And when reduced further, these slow waves become global.

Furthermore, differences and changes have been found in functional connectivity in the resting state; that is, in the connection and neuronal activation patterns of separate brain regions. In this way, when neuromodulation decreases and the first slow waves appear, no changes are observed in this regard. However, when it is reduced further (and slow waves become global), functional connectivity varies and resting-state neural networks merge into a single network widely synchronized.

This tells us that The wake-sleep transition is gradual and depends on chemical and electrical changes in the brain. Multiple neurotransmitters such as GABA, melatonin or adenosine participate in these processes.

What about the sleep-wake transition?

The transition from a dream state to a waking state also arouses great interest. A multitude of chemical processes are also involved in this transition. For example, norepinephrine, serotonin or histamine produce cortical activation and promote states of alertness and the vigil; the activity of their discharge systems decreases during slow-wave sleep and is further reduced during REM sleep.

Particularly, recent findings have highlighted the role of hypocretin in sleep control. An investigation published in the journal Nature has proven how this substance is fundamental in the transition from sleep to wakefulness and the role it plays in the stability of awakening.

Hypocretin-producing neurons (a group of brain cells) are located in the lateral hypothalamus. It has been seen that the electrical activity that arises from them promotes awakening and it is essential to maintain wakefulness.

In the study, direct photostimulation was applied to these neurons in the brains of mice, proving that it increased the probability of transition from slow wave sleep or REM sleep to wakefulness. Besides, stimulation at higher frequencies reduced the latency to wakefulnesscausing you to wake up more quickly.

By confirming the direct relationship between the activity of these neurons and the sleep-wake transition, it is suggested that they also could be involved in narcolepsy. And the loss of function of these neurons would make it impossible to sustain the stability of excitation, causing sudden sleep attacks.

Some substances, such as norepinephrine, serotonin and histamine, act on wakefulness processes.

Prevention of sleep disorders

The above are just some of the findings we have about sleep regulation, but more research is still needed in this regard.

Understanding brain functioning during wakefulness and rest facilitates the design of more effective interventions to address sleep disorders; Given the large number of people affected by them globally, this is a priority task.

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