WHAT HAPPENS WHEN WE WAKE UP -NEUROLOGICALLY

The reticular activating system (RAS) of the brainstem

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—a core of neurons clusters called nuclei—and the thalamus perched on top of the brainstem are gateways that determine what stimuli from the sense organs and the lower body are allowed to pass through and come to a person’s conscious attention. A daily period of sound sleep is vital to human function and survival, so it behooves the hindbrain (“lower brain”) to protect the forebrain (“higher brain,” so to speak) from being overattentive to stimuli to the point that sleep is interrupted by trivia. These two hindbrain systems raise the body’s sensory thresholds to let the forebrain sleep.

When the individual has had enough sleep for one day, the hypothalamus somehow receives feedback to that effect (perhaps sensing it from falling levels of adenosine, an ATP-derived sleep inducer), and it begins to secrete neuropeptides called orexins.

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 Orexins stimulate the RAS, reducing the signal blockade. The RAS responds by stimulating the thalamus just above it. Elevated electrical activity can be detected in the thalamus in sleep-study subjects as the hypothalamic rooster crows and the thalamus yawns and stretches its arms. That wave of activity spreads through the cerebral cortex, cerebral awareness of environmental stimuli increases, and the person wakes up.

Multiple excitatory neurotransmitters (including histamine) are involved in this signaling pathway from RAS to thalamus to cerebral cortex. That’s why blocking the pathway with antihistamines makes us sleepy. In narcolepsy, the sleep disorder in which a person has an overpowering drive to fall asleep during normal hours of daytime activity, the orexin level is abnormally low. Narcoleptics seem short on the neuropeptide that tells the cerebrum “wake up” and “stay awake.” Narcolepsy seems to be an autoimmune disease in which misguided self-antibodies destroy the neurons that produce orexins.

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 In all the trillions of neurons in the brain, we have only 50,000 to 80,000 that produce orexins—precious few, if we start to lose them to immune attack. The word orexin literally means “appetite” and refers to another function of these peptides: to stimulate hunger.

For all that, neuroscientists still know very little about the functions of sleep. It is unclear, for example, why it’s so important for people to lose consciousness for one-third of their lives. Why can’t quiet bed rest alone, but remaining conscious, serve the presumed restorative function of sleep? Sleep researcher J. Allen Hobson quipped about how little we know, “The only known function of sleep is to cure sleepiness.”

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Scientists have recently discovered a brain-cleansing, lymphatic-like system called the glymphatic system. Elsewhere in Quora, I’ve written more about that and how we seem to get a nighly “brainwashing” while we’re asleep.

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 For some reason, it seems necessary to lose consciousness for this system to rev up and give the brain its daily houselkeeping.