Do you think you got enough sleep this past week? Can you recall the last time you woke up without an alarm clock feeling refreshed, not needing caffeine? If the answer to either of these questions is “no,” you are not alone. Two-thirds of adults throughout all developed nations fail to obtain the recommended eight hours of nightly sleep.

I doubt you are surprised by this fact, but you may be surprised by the consequences. Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease. Inadequate sleep even moderate reductions for just one week disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic. Short sleeping increases the likelihood of your coronary arteries becoming blocked and brittle, setting you on a path toward cardiovascular disease, stroke, and congestive heart failure. Fitting Charlotte Brontë’s prophetic wisdom that “a ruffled mind makes a restless pillow,” sleep disruption further contributes to all major psychiatric conditions, including depression, anxiety, and suicidality.

Perhaps you have also noticed a desire to eat more when you’re tired? This is no coincidence. Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction. Despite being full, you still want to eat more. It’s a proven recipe for weight gain in sleep deficient adults and children alike. Worse, should you attempt to diet but don’t get enough sleep while doing so, it is futile, since most of the weight you lose will come from lean body mass, not fat.

Add the above health consequences up, and a proven link becomes easier to accept: the shorter your sleep, the shorter your life span. The old maxim “I’ll sleep when I’m dead” is therefore unfortunate. Adopt this mind-set, and you will be dead sooner and the quality of that (shorter) life will be worse. The elastic band of sleep deprivation can stretch only so far before it snaps. Sadly, human beings are in fact the only species that will deliberately deprive themselves of sleep without legitimate gain. Every component of wellness, and countless seams of societal fabric, are being eroded by our costly state of sleep neglect: human and financial alike. So much so that the World Health Organization (WHO) has now declared a sleep loss epidemic throughout industrialized nations.II It is no coincidence that countries where sleep time has declined most dramatically over the past century, such as the US, the UK, Japan, and South Korea, and several in western Europe, are also those suffering the greatest increase in rates of the aforementioned physical dis- eases and mental disorders.

Caffeine, Jet Lag, and Melatonin

How does your body know when it’s time to sleep? Why do you suffer from jet lag after arriving in a new time zone? How do you overcome jet lag? Why does that acclimatization cause you yet more jet lag upon returning home? Why do some people use melatonin to combat these issues? Why (and how) does a cup of coffee keep you awake? Perhaps most importantly, how do you know if you’re getting enough sleep?

There are two main factors that determine when you want to sleep and when you want to be awake. As you read these very words, both factors are powerfully influencing your mind and body. The first factor is a signal beamed out from your internal twenty-four hour clock located deep within your brain. The clock creates a cycling, day night rhythm that makes you feel tired or alert at regular times of night and day, respectively. The second factor is a chemical substance that builds up in your brain and creates a “sleep pressure.” The longer you’ve been awake, the more that chemical sleep pressure accumulates, and consequentially, the sleepier you feel. It is the balance between these two factors that dictates how alert and attentive you are during the day, when you will feel tired and ready for bed at night, and, in part, how well you will sleep.


Central to many of the questions in the opening paragraph is the powerful sculpt- ing force of your twenty-four-hour rhythm, also known as your circadian rhythm. Everyone generates a circadian rhythm (circa, meaning “around,” and dian, derivative of diam, meaning “day”). Indeed, every living creature on the planet with a life span of more than several days generates this natural cycle. The internal twenty- four-hour clock within your brain communicates its daily circadian rhythm signal to every other region of your brain and every organ in your body.

Your twenty-four-hour tempo helps to determine when you want to be awake and when you want to be asleep. But it controls other rhythmic patterns, too. These include your timed preferences for eating and drinking, your moods and emotions, the amount of urine you produce,I your core body temperature, your metabolic rate, and the release of numerous hormones. It is no coincidence that the like lihood of breaking an Olympic record has been clearly tied to time of day, being maximal at the natural peak of the human circadian rhythm in the early afternoon. Even the timing of births and deaths demonstrates circadian rhythmicity due to the marked swings in key life-dependent metabolic, cardiovascular, temperature, and hormonal processes that this pacemaker controls.

Ape Beds, Dinosaurs, and Napping with Half a Brain 

When did life start sleeping? Perhaps sleep emerged with the great apes? Maybe earlier, in reptiles or their aquatic antecedents, fish? Short of a time capsule, the best way to answer this question comes from studying sleep across different phyla of the animal kingdom, from the prehistoric to the evolutionarily recent. Investigations of this kind provide a powerful ability to peer far back in the historical record and estimate the moment when sleep first graced the planet. As the geneticist Theodosius Dobzhansky once said, “Nothing in biology makes sense except in light of evolution.” For sleep, the illuminating answer turned out to be far earlier than anyone anticipated, and far more profound in ramification.

Without exception, every animal species studied to date sleeps, or engages in something remarkably like it. This includes insects, such as flies, bees, cock- roaches, and scorpions;I fish, from small perch to the largest sharks;II amphibians, such as frogs; and reptiles, such as turtles, Komodo dragons, and chameleons. All have bona fide sleep. Ascend the evolutionary ladder further and we find that all types of birds and mammals sleep: from shrews to parrots, kangaroos, polar bears, bats, and, of course, we humans. Sleep is universal.

Even invertebrates, such as primordial mollusks and echinoderms, and even very primitive worms, enjoy periods of slumber. In these phases, affectionately termed “lethargus,” they, like humans, become unresponsive to external stimuli. And just as we fall asleep faster and sleep more soundly when sleep-deprived, so, too, do worms, defined by their degree of insensitivity to prods from experimenters.

How “old” does this make sleep? Worms emerged during the Cambrian explosion: at least 500 million years ago. That is, worms (and sleep by association) pre- date all vertebrate life. This includes dinosaurs, which, by inference, are likely to have slept. Imagine diplodocuses and triceratopses all comfortably settling in for a night of full repose!

Regress evolutionary time still further and we have discovered that the very simplest forms of unicellular organisms that survive for periods exceeding twenty-four hours, such as bacteria, have active and passive phases that correspond to the light-dark cycle of our planet. It is a pattern that we now believe to be the precursor of our own circadian rhythm, and with it, wake and sleep.

Many of the explanations for why we sleep circle around a common, and per- haps erroneous, idea: sleep is the state we must enter in order to fix that which has been upset by wake. But what if we turned this argument on its head? What if sleep is so useful so physiologically beneficial to every aspect of our being that the real question is: Why did life ever bother to wake up? Considering how biologically damaging the state of wakefulness can often be, that is the true evolutionary puzzle here, not sleep. Adopt this perspective, and we can pose a very different theory: sleep was the first state of life on this planet, and it was from sleep that wake fulness emerged. It may be a preposterous hypothesis, and one that nobody is taking seriously or exploring, but personally I do not think it to be entirely unreasonable. Whichever of these two theories is true, what we know for certain is that sleep is of ancient origin. It appeared with the very earliest forms of planetary life. Like other rudimentary features, such as DNA, sleep has remained a common bond uniting every creature in the animal kingdom. A long lasting commonality, yes; however, there are truly remarkable differences in sleep from one species to an- other. Four such differences.

Another remarkable difference in sleep across species is composition. Not all species experience all stages of sleep. Every species in which we can measure sleep stages experiences NREM sleep the non dreaming stage. However, insects, amphibians, fish, and most reptiles show no clear signs of REM sleep the type associated with dreaming in humans. Only birds and mammals, which appeared later in the evolutionary timeline of the animal kingdom, have full blown REM sleep. It suggests that dream (REM) sleep is the new kid on the evolutionary block. REM sleep seems to have emerged to support functions that NREM sleep alone could not accomplish, or that REM sleep was more efficient at accomplishing.

Yet as with so many things in sleep, there is another anomaly. I said that all mammals have REM sleep, but debate surrounds cetaceans, or aquatic mammals. Certain of these ocean-faring species, such as dolphins and killer whales, buck the REM sleep trend in mammals. They don’t have any. Although there is one case in 1969 suggesting that a pilot whale was in REM sleep for six minutes, most of our assessments to date have not discovered REM sleep or at least what many sleep scientists would believe to be true REM sleep in aquatic mammals. From one perspective, this makes sense: when an organism enters REM sleep, the brain paralyzes the body, turning it limp and immobile. Swimming is vital for aquatic mammals, since they must surface to breathe. If full paralysis was to take hold during sleep, they could not swim and would drown.

The mystery deepens when we consider pinnipeds (one of my all-time favorite words, from the Latin derivatives: pinna “fin” and pedis “foot”), such as fur seals. Partially aquatic mammals, they split their time between land and sea. When on land, they have both NREM sleep and REM sleep, just like humans and all other terrestrial mammals and birds. But when they enter the ocean, they stop having REM sleep almost entirely. Seals in the ocean will sample but a soupçon of the stuff, racking up just 5 to 10 percent of the REM sleep amounts they would normally enjoy when on land. Up to two weeks of ocean-bound time have been documented without any observable REM sleep in seals, who survive in such times on a snooze diet of NREM sleep.

Changes in Sleep Across the Life Span

Through speech or song, expecting parents will often thrill at their ability to elicit small kicks and movements from their in utero child. Though you should never tell them this, the baby is most likely fast asleep. Prior to birth, a human infant will spend almost all of its time in a sleep-like state, much of which resembles the REM-sleep state. The sleeping fetus is therefore unaware of its parents’ performative machinations. Any co-occurring arm flicks and leg bops that the mother feels from her baby are most likely to be the consequence of random bursts of brain activity that typify REM sleep. Adults do not—or at least should not—throw out similar nighttime kicks and movements, since they are held back by the body-paralyzing mechanism of REM sleep. But in utero, the immature fetus’s brain has yet to construct the REM-sleep muscle-inhibiting system adults have in place. Other deep centers of the fetus brain have, however, already been glued in place, including those that generate sleep. Indeed, by the end of the second trimester of development (approximately week 23 of pregnancy), the vast majority of the neural dials and switches required to produce NREM and REM sleep have been sculpted out and wired up. As a result of this mismatch, the fetus brain still generates formidable motor commands during REM sleep, except there is no paralysis to hold them back. Without restraint, those commands are freely translated into frenetic body movements, felt by the mother as acrobatic kicks and featherweight punches. At this stage of in utero development, most of the time is spent in sleep. The twenty-four-hour period contains a mishmash of approximately six hours of NREM sleep, six hours of REM sleep, and twelve hours of an intermediary sleep state that we cannot confidently say is REM or NREM sleep, but certainly is not full wake- fulness. It is only when the fetus enters the final trimester that the glimmers of real wakefulness emerge. Far less than you would probably imagine, though just two to three hours of each day are spent awake in the womb. Even though total sleep time decreases in the last trimester, a paradoxical and quite ballistic increase in REM-sleep time occurs. In the last two weeks of pregnancy, the fetus will ramp up its consumption of REM sleep to almost nine hours a day. In the last week before birth, REM-sleep amount hits a lifetime high of twelve hours a day. With near insatiable appetite, the human fetus therefore doubles its hunger for REM sleep just before entering the world. There will be no other moment during the life of that individual—pre-natal, early post-natal, adolescence, adulthood, or old age—when they will undergo such a dramatic change in REM- sleep need, or feast so richly on the stuff.

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