In Chapter 16, Attia focuses on sleep because “poor sleep can take a wrecking ball to both your long-term health and your ability to function day-to-day” (352). Despite its relationship to health, sleep remains undervalued in most industrialized nations, including the US. Attia hopes to change perspectives on sleep.

He first addresses the question “How long do we need to sleep” (354)? Based on studies, including those conducted in dark caves, humans need around eight hours of sleep per night. Sleep-deprived individuals underestimate how too little sleep impacts their performance and energy levels. They adapt to this state. Once they start getting good quality and quantity sleep, however, they immediately see a difference. Too much sleep (excess of 11 hours) has similar adverse health effects as too little sleep.

Attia turns to how sleep impacts the body. Lack of sleep impacts our metabolic health. It gives seemingly healthy people, like Navy SEALS, “old-man blood,” which is when their inflammatory and hormone markers are decades older than their actual age. Poor sleep causes insulin resistance, wrecking metabolism. Lack of sleep also causes stress, which harms metabolic health. Stress at night while an individual sleeps can elevate glucose levels.