Over 2.6 million adults over the age of 65 develop delirium each year in the United States (US). Delirium is associated with a significant increase in mortality and the US health care costs associated with delirium are estimated at over $164 billion annually. Despite the prevalence of the condition, the molecular pathophysiology of delirium remains unexplained, limiting the development of pharmacotherapies."

Source : The Neurohospitalist, Volume 14, Issue 2 ^{[ paywalled ]}

Delirium (a.k.a. encephalopathy, acute confusional state, acute brain dysfunction, acute brain failure, and altered mental status) is an extremely widespread disturbance of Consciousnessplugin-autotooltip__plain plugin-autotooltip_bigConsciousness

inexplicable

Does an ant carrying a piece of leaf across the forest floor ‘know’ it’s doing so? Is it conscious? If science could construct a machine as complex as the human brain, would it ‘know’ it existed? Will it oneday be possible to explain consciousness via the laws of physics? which has been recognized for millennia. It has been defined as :

• A. Disturbance in attention and awareness. This is a required symptom and involves easy distraction, inability to maintain attentional focus, and varying levels of alertness.
• B. Onset is acute (from hours to days), representing a change from baseline mentation and often with fluctuations throughout the day
• C. At least one additional cognitive disturbance (in memory, orientation, language, visuospatial ability, or perception)
• D. The disturbances (criteria A and C) are not better explained by another neurocognitive disorder
• E. There is evidence that the disturbances above are a "direct physiological consequence" of another medical condition, substance intoxication or withdrawal, toxin, or various combinations of causes

As the research paper cited above points out, although it's well known that a very wide range of factors can lead to delirious states (drugs, stress, fever, dementia, lack of oxygen, shock etc. etc.) the specific neurological mechanisms which initiate and maintain the state are unknown.

Also see : Postoperative cognitive dysfunction - POCDplugin-autotooltip__plain plugin-autotooltip_bigPostoperative cognitive dysfunction - POCD

"Postoperative cognitive dysfunction (POCD) is a decline in cognitive function (especially in memory and executive functions) that may last from 1–12 months after surgery, or longer. In some cases, this disorder may persist for several years after major surgery. POCD is distinct from emergence delirium. Its causes are under investigation and occurs commonly in older patients and those with pre-existing cognitive impairment. and Dreamingplugin-autotooltip__plain plugin-autotooltip_bigDreaming

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"The content and purpose of dreams are not yet understood, though they have been a topic of speculation and interest throughout recorded history.

Source : Wikipedia"

Not only is the function of dreaming unknown (if there is one), but there isn't as yet an agreed definition of what it is.