Why We Sleep

By Matthew Walker (2017)

In the Northern Hemisphere, the switch to daylight savings time in March results in most people losing an hour of sleep opportunity. Should you tabulate millions of daily hospital records, as researchers have done, you discover that this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day. Impressively, it works both ways. In the autumn within the Northern Hemisphere, when the clocks move forward and we gain an hour of sleep opportunity time, rates of heart attacks plummet the day after.

⚠️ See: Matthew Walker’s “Why We Sleep” Is Riddled with Scientific and Factual Errors (Alexey Guzey)

My Notes

Lack of sleep has worse effects than the equivalent absence of food or exercise. It increases the risk of:


All animals sleep, it’s a necessary function for health. Even migrating birds, swimming whales…

People have a strong genetic predisposition to sleep and wake early, or late, but every adult needs eight hours of sleep every night. People naturally find it more difficult to sleep as they get older. This does not mean they need to sleep less.

Your body cannot recover lost sleep.

Sleep is induced through the 24-hour circadian rhythm (reset by light) and a build-up (“sleep pressure”) while awake of adenosine (reset with sleep). Caffeine blocks adenosine receptors.

All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours.

After being awake for nineteen hours, people who were sleep-deprived were as cognitively impaired as those who were legally drunk.

After ten days of just seven hours of sleep, the brain is as dysfunctional as it would be after going without sleep for twenty-four hours.

Vehicle accidents caused by drowsy driving exceed those caused by alcohol and drugs combined.

If you don’t sleep the very first night after learning, you lose the chance to consolidate those memories.

The World Health Organization has officially classified nighttime shift work as a “probable carcinogen.”

Lack of sleep causes death.

Any adult sleeping an average of 6.75 hours a night would be predicted to live only into their early sixties.