Hypothalamus

The hypothalamus is a part of the brain made of different nuclei that serve a variety of functions, such as to control fatigue, sleep, and circadian rhythms.

The hypothalamus regulates sleep by a homeostatic mechanism, wherein pressure is built up during waking hours in the form of adenosine accumulation, which is relieved at sleep onset (or when caffeine is ingested).

According to another theory, the hypothalamus can shut off the arousal system, thereby bringing on sleep.

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The hypothalamus has an anterior as well as posterior areas. When the anterior portion is damaged, it leads to insomnia, while sleepiness results when the posterior portion is damaged.

This suggests that sleep may be a result of activation of the anterior hypothalamus and inhibition of the posterior hypothalamus.

The Anterior Hypothalamus and the Ventrolateral Preoptic Nucleus (VLPO)

The ventrolateral preoptic nucleus (VLPO), which is also known as the intermediate nucleus of the preoptic area (IPA), is located in the anterior hypothalamus.

The VLPO is part of the brain’s sleep-promoting nuclei. One of its functions is to control states of arousal, sleep, and transitions between these two states.

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How does it accomplish that?

During sleep, it becomes activated (by the accumulation of adenosine) and releases a neurotransmitter called GABA, or gamma-Aminobutyric acid, which inhibits the wakefulness promoting ascending arousal system.

During wakefulness, on the other hand, the VLPO is deactivated by the neurotransmitters norepinephrine and acetylcholine.

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The Posterior Hypothalamus

The posterior hypothalamus may activate the ascending arousal system, thereby promoting wakefulness.

In narcolepsy, this system is damaged, resulting in excessive sleepiness during the day as well as frequent nocturnal awakenings.