Evolution has altered the human brain to adapt to stress. Our ancient ancestors were vulnerable to many dangerous predators, and when confronted with such threats, they experienced a fight-or-flight response.

This natural defense mechanism led to a spike of the hormone cortisol, which along with other stress hormones accelerates the heart rate and heightens mental acuity.

These effects provided a survival advantage to ancient humans. The modern brain has retained those ancient neural responses, even though we no longer face the same kinds of immediate predatory threats on a daily basis.

Instead, we experience ongoing, low levels of stress from traffic, noise, deadlines, and other persistent challenges of modern life.

Those factors can lead to chronic elevation of stress hormones, which can actually damage brain cells, impair memory, and worsen mood.