Longevity Series: Sleep – Part I

As a society, we have a severe problem with sleep: according to the CDC, one in three US adults is chronically underslept. Sleep has become stigmatized as laziness and sloth, and bragging about how little sleep some individuals need has become a badge of honor–a huge issue, as recent studies have demonstrated that sleep is foundational to our mental and physical health and performance.

The Consequences of Lack of Sleep on Our Health

Sleep disturbance increases our risk of cardiovascular disease, doubles our risk of cancer, interrupts our immune system’s ability to heal, causes diabetes, and impairs our motor and cognitive functions. Lack of sleep derails our ability to tackle problems and causes anxiety and mental illness, driving increased risk of dementia and Alzheimer’s.

Sleep scientist Matthew Walker, Ph.D., even postulates that we didn’t evolve sleep, but the default state of life on the planet was sleep, and it was from sleep that wakefulness emerged. It’s a tough hypothesis to test, but tenable. All mammals need a substantial amount of sleep, but humans are serial offenders in trying to duck our biological need for it: over the span of 70 years, we’ve cut our sleep by 25 percent, with devastating consequences.

To mark World Sleep Day on March 17, here’s the first of a two-part story on why we need to sleep better, based on insights shared by Walker on The Drive and in his seminal book Why We Sleep (resource notes below).

What is Sleep, and Why Do We Need So Much of It?

We experience two stages of sleep. One is called non-rapid eye movement sleep, or non-REM sleep. The other is rapid eye movement, or REM sleep, when most dreaming occurs. Just before we enter REM, our heart rate drops, and our brain stem sends a signal to paralyze our voluntary muscles so we don’t physically act out our dreams. Non-REM sleep is subdivided into four separate stages, increasing according to the depth of sleep: stages one and two are the lightest stages of sleep; stages three and four of non-REM sleep are deeply restorative stages of sleep.

While we sleep, we switch between REM and non-REM every 90 minutes, forming a standard cycling architecture of sleep, or hypnogram. When we close our eyes, our brain stops processing the outside world, and our eyes roll slowly in their sockets–the first sign we’re falling asleep. Then, we descend into stages one and two, the lighter stages of non-REM sleep. After around 20 minutes, we go deeper into stages three and four of non-REM sleep, when our brain waves become slower in frequency and higher in amplitude as hundreds of thousands of neurons fire together and then stop. It’s at this stage that we get information transfer. Much in the same way that long-wave radio signals can be picked up far from the source, our brain in this deep state of non-REM sleep can carry more information.

Another reason we sleep: it gives our brain a chance to effectively power-wash away waste. In 2012, Rochester University reported that it had determined that the brain has a type of sewage system, the glymphatic system. When we go into a deep sleep at night, this sewage system kicks into high gear, and its glial cells, which surround brain cells, shrink in size by up to 200 percent, leaving plenty of space for cerebrospinal fluid to flush out the metabolic effluvia of wakefulness, much of which hampers our ability to function and would otherwise quickly negatively impact our health. Beta-amyloid, one of the two core proteins that underlie your risk for the development of Alzheimer’s, comprises some of this toxic garbage–more on the connection between Alzheimer’s and sleep below.

Why So Many of Us Don’t Get Good Sleep

There are four pillars of sleep:

  1. Regularity–the consistency of our sleep schedule
  2. Continuity–how often we wake up
  3. Quantity–how many hours we sleep
  4. Quality–the electrical signature of your sleep

It’s not uncommon to hear leaders say they only need five or six hours of sleep a night–especially in competitive cities where entrepreneurs and executives might hit the hay after a late dinner and wake up at 5.30 a.m. to get a jump on the day. While cutting sleep hours will always negatively impact our health, the nature of that impact is also influenced by when we cut those two hours, as our brains have more of an appetite for REM and non-REM sleep at different stages of the night. In the first half of the night, most 90-minute sleep cycles comprise non-REM sleep. In the latter half, it’s mainly REM sleep. If we sleep from 9 p.m. and wake at 3 a.m. to hit the gym, we’ll spend longer in a deep, non-REM state and lose more of the rich REM sleep–early risers that short-change their total sleep by 25 percent are losing up to 70 percent of all REM sleep. Night owls who sleep from midnight to 6am (or later) will get more REM and less non-REM sleep.

REM is important because it stimulates the areas of the brain that help with learning and emotional processing and is associated with the increased production of proteins. During the deep stages of non-REM sleep, the body builds bone and muscle, repairs and grows tissues, and shores up the immune system. Different types of memory also rely on the different stages of sleep. We need deep, non-REM sleep to save memories accrued during the day–if we go to bed before midnight and get five hours of sleep, we’ll likely retain most of the information garnered during waking hours. During the second half of the night, there’s a higher concentration of stage two non-REM sleep, which prepares our brains to lay down new memories the following day.

We need both non-REM and REM sleep (although studies have shown that, purely from a mortality standpoint, being deprived of REM sleep is just as fatal as being deprived of sleep entirely).

The Dangers of Poor Sleep: An Overview

The most significant clinical detriment of a reduction in REM is a dip in mental health, including depression, anxiety, and suicidality, which seems to perpetuate the negative spiral: if you can’t sleep, you become more anxious, making it harder to sleep. In teenagers, one of the strongest predictors of thoughts of taking their own life and suicide attempts is insufficient sleep.

Sleep, especially REM sleep, boosts our emotional wellbeing and acts as overnight therapy. REM sleep resets or recalibrates the networks in the prefrontal cortex, particularly the middle part, which acts like a top-down regulator of our emotions. At the same time, the amygdala generates strong, impulsive, and negatively aversive reactions.

In toddlers and young children, the prefrontal cortex is still developing, so they are more prone to tantrums–which is why all parents put such a huge emphasis on essential naptime because children (and adults, for that matter) are usually extra cranky when they don’t get enough shut-eye. A lack of sleep has a faster reduction in health than not eating or exercising for a period: for every five percent reduction in REM sleep, there’s a corresponding increase of 13 percent in all-cause mortality.

Effects of Poor Sleep: Disease and Illness

A lack of sleep correlates strongly with cardiovascular disease. A study tracked a group of otherwise healthy, middle-aged adults with no sign of coronary artery disease for five years, looking closely at how much sleep they were getting. When the study began, subjects had no signs of coronary artery calcification; at the end, the individuals who slept five hours or fewer a night had an increased risk factor of 200 percent to 300 percent. When we’re sleep deprived, our sympathetic nervous system, the branch responsible for our fight or flight impulse, starts to ratchet up, more adrenaline is released, cortisol levels and blood pressure spike, and growth hormone drops.

Walker posits that these changes in cortisol and growth hormone alone may increase the risk of cardiovascular disease as both factors are tied to atherosclerotic pathology. He points to evidence to which we can all relate: the effect of daylight savings on 1.6 billion people across 70 countries every year. When we lose an hour of sleep in the spring, there’s a 24 percent increased risk of heart attacks the following day. In the fall, when we gain an hour of sleep opportunity, there’s a corresponding 21 percent reduction in heart attacks.

This same trend is seen for suicide attempts and completions, and for federal judges handing out harsher sentences–it’s a no-brainer that losing an hour of sleep hugely impacts our mood and mental health. Car accidents follow a similar profile: people that deprive themselves of sleep over time, for example by sleeping five hours a night for a couple of months, as well as missing sleep in a 24-hour period, are also more likely to experience microsleeps behind the wheel and lose control of their vehicle.

Lack of sleep causes a dip in our immune system and a diminished ability for T cells to identify threats. A UCLA study concluded that partial sleep deprivation reduces natural killer cell activity in humans–when healthy adults were limited to four hours of sleep per night, scientists noted a 70 percent drop in their natural killer cells, a type of lymphocyte in the same category as T and B cells that identify and destroy foreign elements like cancer cells. There’s a significant link between sleep and cancers of the bowel, prostate, and breast (three out of the top four commonly occurring cancers). The WHO has classified night shift work as a “probable carcinogen.” A study using mice by David Gozal at the University of Chicago found that the tumors in the underslept group were 200 percent larger than those in mice who slept normally; the cancer in underslept mice had also metastasized.

Oxidative stress is concurrent with insufficient sleep, which causes domino-like cascades of effects resulting in neuronal death. One area in the brain most sensitive to neuronal death is the hippocampus, a critical memory center and one of the first structures to be damaged by Alzheimer’s disease. It’s part of why the sort of the phenotype of Alzheimer’s is forgetfulness and other memory problems. Walker says the effect is causal: sleep may be one of the most significant lifestyle factors determining your risk ratio for Alzheimer’s disease.

Vascular disease and insulin resistance also feed into a mechanism of neuronal energy deprivation–wakefulness is low-level brain damage, and we need sleep to avert it. Walker says wakeful sleep is the price we pay for wakefulness–good sleep is more about sleep quality than quantity.

Join us next week when we’ll look at the effects of poor sleep on metabolism, weight, performance, fertility, and genetics. We’ll also share how caffeine, alcohol, THC, and CBD impact sleep–and share tips from Registered Dietician and Nutritionist Brittany Kearney on how to get better sleep.

Mark your calendar for Brittany’s next event, Longevity 101, at Menlo Park on 03.30.23.


  • Why We Sleep, by Matthew Walker, Ph.D., professor of neuroscience and psychology at the University of California Berkeley and the founder and director of the Center for Human Sleep Science. Walker is also a sleep scientist at Google, supporting the scientific exploration of sleep and its impact on health and disease.
  • Episodes dedicated to sleep on The Drive, hosted by Peter Attia, MD, the founder of medical practice Early Medical, which applies principles of Medicine 3.0 to patients aiming to lengthen and improve their lifespan