An ancient story of life, rest and renewal
At night, while the visible world grows quiet, life beneath the surface changes pace. From the ocean depths where jellyfish drift in ancient rhythms to the quiet chambers of the human mind, sleep remains one of life’s oldest and most persistent mysteries. It is a dangerous bargain—to lower one’s guard, to become slow to react, to surrender vigilance in a world filled with risk. Yet across evolutionary ages, life has repeatedly evolved this strange, vital habit. In an era of constant illumination and endless digital demands, Why We Sleep explores the deep biology, history and necessity of the nightly pause—reminding us that to rest is not to step away from life, but to participate in its most enduring rhythm of renewal.
“Sleep that knits up the ravelled sleave of care,
The death of each day’s life, sore labour’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast.”
— William Shakespeare, Macbeth, Act II, Scene II
At night, the forest does not fall silent in the way we imagine.
Some creatures are only beginning their work: bats stir in hollow trees, owls slip soundlessly between the trunks, and small night-farers emerge from the leaf litter. Others withdraw from the darkness: birds settle into dense foliage, a deer beds down in a sheltered thicket, and insects fold themselves beneath bark, stone or soil.
In the canopy, on the forest floor, and in the hidden chambers below it, life changes pace.
To sleep is to lower one’s guard. It is to become slower to notice danger, slower to run, slower to respond. A sleeping creature cannot search for food, defend its territory, attend to its young or watch for predators in quite the same way as an alert one can.
And yet life has kept this strange habit — through oceans and deserts, forests and grasslands, nests and caves, villages and cities.
Sleep survives, perhaps, because it is one of life’s oldest ways of repairing itself.
In our illuminated rooms, the invitation to stillness is easily postponed: by one more message, one more episode, one more small task, or one more restless scroll. Yet the body remains older than the screen. It continues to ask for darkness, for safety, for a release from vigilance.
Sleep is an active and restorative part of being alive — a time in which the body attends to itself and the mind reorganises experience, leaving us renewed and ready for the day ahead.
A mystery older than dreams
Scientists do not know exactly when sleep first appeared in the history of life, nor do they have one final answer to the question of why it evolved.
Human sleep — with its dreams, deep stages, rapid eye movement and private interior worlds — is only one expression of a far wider mystery, and should not be taken as the measure of every creature’s rest.
In biological research, sleep is generally recognised through a cluster of features: recurring behavioural stillness, reduced responsiveness to the surroundings, a characteristic posture or place, the ability to wake when something sufficiently important happens, and a tendency to compensate after sleep has been withheld.
That last quality — sleep rebound — is especially revealing. An animal that has lost sleep commonly sleeps longer or more intensely later, suggesting that sleep is a biological need rather than a convenient pause.
Among mammals and birds studied in detail, sleep is widespread, though its duration, depth and timing vary enormously. Across much of the rest of the animal kingdom, the evidence is less complete, and scientists remain cautious about assuming that every creature sleeps in precisely the same sense.
Even so, the picture has widened remarkably.
Reptiles, fish, insects and other animals show sleep-like states — periods of reduced activity and responsiveness that can be followed by compensatory rest. In 2017, researchers reported evidence of a sleep-like state in the upside-down jellyfish Cassiopea, an animal with a diffuse nerve net rather than a centralised brain. When its nighttime quiescence was disturbed, the jellyfish subsequently showed reduced activity and responsiveness, suggesting a homeostatically regulated sleep-like state.
A jellyfish does not sleep as a person sleeps. Yet such findings suggest that the alternation between activity and quietness may be far older than dreams or complex thought.
Some researchers believe that sleep may have grown from ancient cellular and circadian rhythms — the deep patterns through which living things respond to light and darkness, manage energy and alternate between activity and restoration. In humans, sleep is shaped by two interacting processes: circadian timing and the homeostatic pressure that builds with time awake.
Before sleep became a dream, it may have been a rhythm.
Who sleeps, who sleeps briefly, and who sleeps by halves?
The sleeping world is not uniform.
Every species has had to negotiate the same difficult bargain: how to obtain the benefits of rest without becoming too vulnerable while resting.
Most well-studied mammals and birds sleep, but they do so in dramatically different ways. Some take long, consolidated periods of rest. Others sleep in short fragments. Some sleep in concealed places, some in groups, and some with a part of the brain still keeping watch.
Dolphins offer one of the most remarkable examples. Several dolphin species can engage in unihemispheric slow-wave sleep: one hemisphere of the brain rests while the other remains awake enough to support breathing, movement and vigilance. Related forms of unihemispheric sleep occur in several marine mammals and birds.
Migratory birds can also reduce sleep dramatically during demanding journeys. Their capacity is astonishing, but it reflects a specialised adaptation to a particular moment in the life of a species — not a lesson in human self-denial.
The differences in ordinary sleep duration are just as striking. In one field study, two free-ranging African elephant matriarchs slept for only about two hours a day; little brown bats, by contrast, may spend close to twenty hours asleep. These are solutions shaped by body, habitat, diet, danger and way of life.
The animal kingdom thus reveals an immense variety of ways of resting. Sleep can be shaped by parenthood, food scarcity, changing seasons, predation, migration and the availability of shelter. Some animals also enter hibernation — a deeper energy-conserving state related to, but distinct from, ordinary sleep.
Sleep belongs to the whole architecture of life.
What sleep may have given life
At the beginning of a human life, sleep occupies an extraordinary share of the day. Infants sleep far more than adults, as though development itself requires long periods in which the visible world is set aside.
We keep looking for the single purpose of sleep. The evidence points in a different direction: sleep seems to serve several needs at once.
Sleep is thought to support the maintenance and functioning of the nervous system. It is closely associated with learning and memory: experiences are not simply stored unchanged, but may be stabilised, integrated with earlier knowledge, strengthened or allowed to fade.
During sleep, the brain does not simply put the day away. It appears to work through what we have experienced, weaving some new information into older memories and allowing other impressions to recede. In that sense, sleep may prepare us not only to remember yesterday, but to meet tomorrow differently.
In human beings, sleep is also closely intertwined with emotion. Sleep loss can affect mood and emotional regulation, while adequate sleep helps provide a more stable foundation for meeting the following day.
There is also the question of timing. Sleep can help an animal occupy a temporal niche — a way of life organised around the hours in which it is safest or most effective to be active. A small animal may be safer concealed in daylight and active under cover of darkness; another may feed in the morning and withdraw during the hottest hours.
Sleep and wakefulness are ways of arranging life around opportunity, climate, predators, food and the limits of the body. They are about more than fatigue.
Taken together, these functions make sleep look less like one invention than a set of old solutions gathered into the same daily pause.
The quieter work of the mind
For human beings, the significance of sleep is felt not only in the body but also in the texture of the following day.
The sleeping mind seems to move through a private theatre — sometimes vivid, sometimes absurd, sometimes emotionally exact in ways waking language cannot manage.
Science can describe sleep stages and investigate the connections between sleep, memory, emotion and brain activity. It does not remove the older human sense that dreams belong to a borderland between what we know and what we have not yet understood.
Ancient traditions often treated sleep as a threshold rather than a blank interval. The Talmud puts it memorably: sleep is “one-sixtieth of death,” and dreaming is “one-sixtieth of prophecy.” The image captures something real — a nightly surrender of ordinary consciousness, and a strange return that follows in the morning.
A good night’s sleep does not solve every problem. Grief does not disappear by morning. A difficult decision may remain difficult, and the world’s injustices do not soften merely because one has rested.
Yet sleep can change the ground on which we meet such things. A thought that seemed unbearable at midnight can sometimes become more manageable at dawn. A conflict may still need to be addressed, but with less heat. A task may not have become easier, but the mind may have recovered enough patience to begin it.
We wake not as entirely new people, but often as people returned a little more fully to ourselves.
And sometimes, the mind keeps working
There is another mystery associated with sleep.
Sleep does not simply preserve yesterday’s experiences. It can help the brain reorganise them.
Research suggests that memories can be reactivated and integrated during sleep, allowing connections between new experiences and older knowledge to emerge. This may sometimes support problem-solving and creative insight. The precise mechanisms — and the roles played by different stages of sleep — remain subjects of active research.
Perhaps that is why the old advice to “sleep on it” sometimes feels wiser than it sounds.
The problem has not necessarily been solved during the night. But the mind may have returned to it differently.
From firelight to electric light
It is tempting to imagine that people in the past slept in a more natural and peaceful way.
The truth may be more complicated.
Pre-industrial nights could have been shaped by cold, illness, crowding, uncertainty, infant care, physical labour, danger and interrupted sleep. A rural home may have been dark, but not necessarily quiet; a smaller city could still be noisy, crowded and unsafe.
There is little reason to imagine a single golden age of perfect human sleep. Yet the architecture of night has changed.
Historical sources from parts of pre-industrial Europe describe what has become known as “first sleep” and “second sleep”: references to an initial period of sleep followed by some form of wakefulness and, in some cases, another period of sleep.
The practice clearly existed in some places and circumstances. But recent scholarship cautions against treating it as a universal pattern of pre-industrial life or as evidence that consolidated sleep is somehow unnatural. The historical evidence is more varied and ambiguous than the popular version of the story suggests.
In warmer parts of the world, the siesta became another rhythm of rest—a retreat from the midday heat before life resumed in the softer hours of the afternoon.
Fire had already extended the evening long before modern technology. It made room for storytelling, food preparation, conversation, ritual, intimacy, craft and companionship after sunset.
Gas lighting and then electric light extended the night further still. They brought freedom, safety, work, education, entertainment and social life into hours once governed more strictly by darkness.
But every gift changes the shape of life. Once night could be brightened at will, it could also be filled — with commerce, obligation, factory shifts, late travel, entertainment, messaging and an inexhaustible stream of information.
A longer evening can become a shorter night when the morning remains fixed.
Research in communities with differing access to electricity suggests that artificial light can delay sleep onset and shorten sleep, although the effects vary with season, lifestyle and social circumstances. Studies in Toba/Qom communities in Argentina and in villages on Tanna Island, Vanuatu, found later sleep onset and reduced nighttime sleep in groups with access to electric light.
Electricity enlarged the evening. Whether that enlargement becomes enrichment or exhaustion depends on what the newly available hours are asked to hold.
Rural and urban nights
Far from the notions of wholesome rural sleep and damaged urban sleep, real nights are less tidy.
A person’s night depends on housing, income, safety, heat, noise, work schedules, commuting, caregiving, family arrangements, local customs and access to health care — not merely on whether they live in a village or a city.
Rest is unevenly distributed. A quiet, dark room; safety at night; predictable work hours; control over one’s schedule; and relief from long commutes, overcrowding, caregiving or night work are not equally available to everyone.
Before rest becomes another measure of personal discipline, it is worth remembering that sleep is shaped by the conditions in which people live.
Why one person sleeps seven hours and another nine
Sleep has a tendency to become moralised.
Early risers may be praised as disciplined. Late sleepers may be accused of laziness. People who claim to thrive on little sleep are admired as though they have discovered a superior form of character.
Yet sleep is shaped by age, health, genetics, stress, illness, medication, activity, caregiving, work and environment.
Sleep also changes across a lifetime. Infants and children require far more of it than adults; adolescence tends to alter the body clock again; and sleep often becomes lighter or more fragmented in later life.
Two biological forces shape its timing: the circadian rhythm and a homeostatic pressure that builds the longer we are awake.
Chronotype — the tendency to prefer earlier or later sleeping and waking — varies from person to person. It has genetic components, but is also influenced by light exposure, culture, work schedules and household routines.
There is no virtue in needing less sleep than someone else. The more meaningful question is whether one is getting sufficient, restorative sleep for one’s body and life.
What changed sleep — and what may help restore it
The long history of extending the day has now entered the bedroom. Screens can bring the office, the marketplace, the theatre, the social gathering, and the global news cycle to the edge of the pillow. Shift work, long commutes, irregular schedules, and the demands of a twenty-four-hour economy can further unsettle the old rhythm between waking and rest.
Caffeine can extend wakefulness. Nicotine can interfere with rest. Alcohol may make a person feel sleepy at first but can fragment sleep later in the night. Medicines, pain, anxiety, depression, breathing problems, trauma, parenthood and illness can all reshape a person’s nights.
Sleep difficulties do not have a single cause, and there is no single remedy for all of them.
Yet sleep often comes more readily where life allows for a measure of rhythm and retreat: a reasonably regular time for winding down; daylight by day and relative darkness at night; a room that feels safe, quiet and comfortable; and an evening not endlessly extended by work, worry or stimulation.
Across cultures and households, small rituals often mark the passage into rest. A few pages of a book. A bath. Soft music. Prayer. An unhurried conversation. A few handwritten lines.
None of these practices guarantees sleep, nor should they become another standard by which people judge themselves. They matter because they make room for rest.
Persistent insomnia, loud snoring with pauses in breathing, marked daytime sleepiness, or sleep troubles that substantially affect daily life are reminders that some nights require more than patience and good habits; they may deserve professional care.
Seeking such help is not a failure of discipline. It is one more way of taking sleep seriously.
The daily lesson of sleep
Sleep is an exercise in humility.
It reminds even the busiest among us that there is work we cannot do by force. We cannot command attention to remain sharp forever. We cannot keep receiving the world without eventually needing a place to set it down.
This is the ordinary condition of being alive.
Nourishment is often understood as something we take in: food, conversation, beauty, learning, affection. But nourishment also comes from what we stop taking in.
The closed eye is not an empty eye. It is an eye relieved of demand.
Sleep nourishes the body through restoration. It lets attention recover. It helps experience settle into memory. And it can influence emotional processing and regulation.
The world will not become less real because we sleep. Its sorrows will still require courage, attention and action in the morning.
But the night was never meant to make us carry every sorrow, argument, prediction and unfinished task at once.
Perhaps sleep has lasted because life cannot be made entirely of outward motion. Every creature must, in its own fashion, withdraw: into a nest, a burrow, a branch, a darkened room, or the unlit interior of itself.
We have learned many ways to postpone that withdrawal. Yet something in us continues to ask for a different rhythm — not for perfection, but for nourishment. A little darkness. A little safety. A release from vigilance. Time for memory to settle, emotion to soften, and tomorrow to become possible.
To sleep is not to turn away from life. It is to take part in one of life’s most enduring rhythms: the rhythm by which so many living beings pause, restore themselves, and return.
Sources & Further Reading
National Institutes of Health — “The New Science of Sleep: From Cells to Large-Scale Societies.” A broad contemporary review of sleep across species, from its evolutionary history to its regulation and social context.
Nath, R. D., et al. — “The Jellyfish Cassiopea Exhibits a Sleep-like State.” Current Biology, 2017. The landmark study reporting a reversible, low-responsiveness nighttime state in Cassiopea, with compensatory effects after disruption, in an animal with a diffuse nerve net rather than a centralised brain.
Rattenborg, N. C., et al. — “Half-asleep birds choose which eye to open.” Nature, 1999. Research on unihemispheric slow-wave sleep in birds; related studies examine similar states in dolphins and other marine mammals, where one brain hemisphere rests while the other maintains vigilance.
Gravett, N., et al. — “Inactivity/Sleep in Two Wild Free-Roaming African Elephant Matriarchs.” PLOS ONE, 2017. The field study that found approximately two hours of daily sleep in the two free-ranging African elephant matriarchs observed.
Lewis, P. A., Knoblauch, A., and Poe, G. R. — “How Memory Replay in Sleep Boosts Creative Problem-Solving.” Trends in Cognitive Sciences, 2018. A review of how sleep-related memory reactivation and integration may contribute to insight, creativity, and problem-solving.
Palmer, C. A., et al. — “Sleep Loss and Emotion: A Systematic Review and Meta-Analysis.” Psychological Bulletin, 2024. A large systematic review and meta-analysis examining the effects of sleep loss on mood and emotional functioning.
de la Iglesia, H. O., et al. — “Access to Electric Light Is Associated With Shorter Sleep Duration in a Traditionally Hunter-Gatherer Community.” Journal of Biological Rhythms, 2015. The field study of Toba/Qom communities in Argentina found later bedtimes and shorter sleep associated with access to electric light.
Thomas, M. G., et al. — “Sleep and wakefulness in indigenous villages on Tanna Island, Vanuatu.” Sleep, 2016. Research showing how electricity, evening light, and local living conditions interact with sleep timing in small communities.
Williams, S. E., and Da Silva, S. — “Have We Lost Sleep? A Reconsideration of Segmented Sleep in Early Modern England.” Medical History, 2023. A careful re-examination of the popular “first sleep/second sleep” account and the historical evidence behind it.