The theory is that the chemo receptors in the brain were developed from when humans were burrowing animals, which forced our brain to send signals to release adrenaline and cortisol to help us move, scrabble, thrash around, and try to get out of danger while asleep. This explains why people wake up screaming or gasping for air during a nightmare, and why some COVID-19 patients do not feel shortness of breath even when their oxygen levels plummet.