The lost medieval habit of 'two sleeps'

Artificial lighting and modern practices like working late are linked with sleep shifts

An analysis of five adults participating in a German reality program that tasked them with living in a Stone-Age-like settlement for 2.5 months, provided some insights into how modern conditions may have shifted our circadian rhythms. The participants slept about 90 minutes more under these conditions, and went to bed earlier and rose earlier, raising questions about how circadian rhythms, sleep, and sleep quality are affected by modern lifestyles.

Sleep is often viewed individually, when its impact and meaning may be more communal

At any given point, roughly 2 billion people may be asleep on Earth. The typical scientific description of sleep focuses on its individual impact, but the phenomenon shapes whole cultures and eons. This essay prompts a fresh rethinking of the importance of sleep for humanity.

Humans may have evolved to need less sleep

Humans sleep less than most primates—but more efficiently. Chimps sleep 9.5 hours with 5% in REM, while humans get seven hours with 25% in REM. Some predict we may one day need just five hours, but for now, our sleep patterns balance rest and function, shaped by evolution and environmental stability.

Some animals seem to live well on little sleep

African bush elephants typically sleep for about 2 hours per 24-hour day, coming in well under a human's recommended seven-hour minimum. Other animals also appear to skimp on sleep, by human standards, without the consequences humans associate with being sleep deprived. This academic article details the sleeping habits of animals that appear to thrive on fewer than 40 winks and explores whether their sleep processes are just built different.

Studies suggest when you sleep is as important as how much you get, and syncing with your circadian rhythms may lead to less fatigue and improved long-term health outcomes

What happens when we sleep

Sleep is a highly specialized and complex process that begins via signals from a region of the brain called the hypothalamus. As explained in this overview video from The Economist, the hypothalamus maintains our circadian rhythm by using light cues from our environment to regulate sleep via a compound known as melatonin. Once asleep, the brain begins the "sleep cycle" and progresses through the four stages of sleep over and over again each night.

Forty to 110 minutes of deep sleep is recommended per night

Adults typically spend 10% to 20% of the night in deep sleep, also known as slow-wave sleep, with the rest of the night spent either in light sleep or REM states.

Chronobiologists study the effect of time on biological cycles

Circadian rhythms were first observed in the 1700s, when French astronomer Jean-Jacques D'Ortous de Mairan put a plant in a dark room and watched it still respond to day and night cycles, setting off further studies about specific light-dark biological cycles, an area of inquiry that continues today with the human "clock proteins" at the molecular level.

What it's like to live with insomnia

The biological process Shakespeare referred to as the chief nourisher of life's feast is necessary for the body and mind to rest and repair from the activities of daily life. As sleep evades the exhausted, sufferers also lose emotional stability, mental clarity, social connections, and more while simultaneously gaining waves of interrupted biological processes that can lead to disease and death.

A 2,000-year history of alarm clocks

Clocks have revolutionized the way we live, structuring our days into abstract, regular units and providing quantification for those who are chronically late. The alarm clock was the next obvious step: clocks which not only tell you the time but...

Wealthy Egyptians slept on woven beds to allow air circulation in hot climates

The history of beds is a fascinating look at economics, social science, and cultural beliefs around what takes place during sleep. This brief survey of the history of bed-making begins roughly 5,000 years ago and runs through ancient Egypt and Rome to modern Europe. Take a look here.

Why you should work 4 hours a day, according to science

Some of history's luminaries worked just four hours a day, including literary great Charles Dickens and biologist Charles Darwin. Observers suggest such a short period forces intense focus sustained by extended rest. In contemporary work, the high premium placed on the number of hours worked can overlook the importance of rest.

Why don't humans hibernate?

Hibernation is a superpower in the animal world that allows certain species to time travel from difficult times to abundance. This essay in Aeon explains this helpful natural process and why humanity's technological advancements may have hindered our ability to participate in the practice. Learn about the rising interest in human hibernation as a result of modern adversity, and if it could potentially become a reality.

The US loses roughly $400B a year in productivity due to sleep deprivation

For an article about a woman who built a business empire telling bedtime stories, the Hustle created this graphic showing the estimated GDP loss associated with chronic insomnia. The United States had a much more significant loss than other countries.

A third of US couples have opted for a 'sleep divorce'

A 2023 survey of roughly 2,000 US adults showed roughly a third of the participants opted to stay in a different room from their partner at night. Reasons for the so-called "sleep divorce" varied, from snoring issues to offset schedules. The survey, run by the American Academy for Sleep Medicine, also revealed the most sleep divorces within millennials. See the study for more on this change in sleep habits.

Babies don't develop a circadian rhythm right away

Newborns don't develop circadian rhythm until they are a few months old, which helps to explain their erratic sleep schedules. When they're around three months old they begin to release melatonin, which influences sleep, and around the same time they develop more regular sleeping routines.

Social jetlag—differing weekend and weekday schedules—is physiologically similar to jet lag

Recovering from schedules that drastically differ between weekends and weekdays, something that was first described in academic work in 2006, is a growing concern in public health as more research shows that consistent daily routines are important for human health.

Researchers believe the brain consolidates memories, processes emotions, and plans for the future during dream states

The world's quirkiest time zones

In contrast to telling time by physical signs like the sun or stars, cultural time relies on political agreement and standards. The convention of time zones arose due to transportation and communication advancements, and while many work just fine, some are pretty wild. Chief among those is the time zone of the uninhabited Baker and Howland Islands in the central Pacific, which results in a technical 26-hour day.

How to end bedtime procrastination

Finally done with external obligations, you might plop onto the sofa and dive into a hobby or bit of media, pushing past sleepiness to have some time to yourself and enjoy doing what you want. This is known as "revenge bedtime procrastination," a behavior that feels relaxing but impairs sleep hygiene.

The Benjamin Franklin letter that anticipated daylight saving time

Benjamin Franklin did not invent daylight saving time. But while living in Paris, he did pen a satirical letter to Parisians suggesting they could save money on lamp oil by changing their sleep schedules. Read it in full here.

Why do we sometimes jolt awake before falling asleep?

Around 70% of people are estimated to experience the sensation of falling and jerking awake right as they're about to drift to sleep. That mysterious phenomenon is called hypnic jerking. As common as it is, hypnotic jerking remains disconcerting and hard to explain. While we don't yet have clear answers, peruse this article in Popular Science for some possible explanations as to why this happens.

The word 'vampire' appeared in the English-speaking world in the 1730s

However, earlier mentions of creatures with similar characteristics existed throughout Medieval Europe. For example, people often spoke of revenants—restless dead creatures who tormented the living. Historians believe these stories grew in popularity in part because of influence from the Catholic Church, which was emphasizing the idea of purgatory. These older revenant stories eventually influenced modern vampire folklore.

Six myths about the Middle Ages

The Middle Ages, or medieval era, refers to roughly 500 CE to 1,500 CE. Contemporary opinion perceives this era as "dark" or backward because it was the time between the fall of Rome and the rise of the Renaissance. But much of the era was vibrant, and many myths are just that, including the idea that most people believed the Earth was flat. Watch this video to debunk more myths.

The concept of 'screen time' was invented in 1991

Mother Jones magazine coined the term in an essay called "Primal Screen" about children's television habits. Writer David Engelhardt wrote that they were watching “screens within screens within screens,” and that even babies were averaging an “hour and a half of screen time a day...the typical older child, about four hours."

New Year's Eve celebrations date back to ancient Babylon

Rather than celebrating in winter, the Babylonians honored the vernal equinox, a day in late March with equal amounts of darkness and light that signaled the start of a new year. They celebrated with Akitu, an 11-day festival.

Early farming created population booms—and also major busts

Europe’s ancient embrace of farming took the continent on a demographic roller-coaster ride. Regional booms and busts in human numbers occurred between 8,000 and 4,000 years ago, a new study finds.

The concept of phantom limbs, the perception that amputated limbs are still present, dates back to a French military surgeon in the 16th century.

The Dracula character, now seen as the vampire archetype, was inspired by a seafood-induced nightmare its creator experienced.

After the Boston Tea Party, colonists switched to coffee, which became the main morning drink for revolutionaries.

In 1963, a basement renovation in Turkey revealed a passageway to a hidden, multilevel underground city that once housed as many as 20,000 people

'Silent' fireworks have gained more interest in recent years as a quieter celebratory option.

Some researchers have labeled nomophobia, the fear of being without your phone, as 'the phobia of the 21st century.'

Explore chaos in this simulation of two double pendulums, where small differences in the release angles produce wildly different outcomes.

At the beginning of the 20th century, a group of young men ate standard American food—replete with food additives like borax and formaldehyde—as part of government research to better understand food safety.

Clever Hans was a 1900s horse that was believed to be able to solve math equations, tell time, and read and comprehend German, until a psychology student discovered how he got his answers.

Cultural norms about how to handle the bodies of loved ones in communities in West Africa have hampered Ebola responses.

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.

Before humans, the Soviet Union and the United States sent animals—including apes, dogs, and cats—to space during the Space Race.

In its famous catalog, Sears sold $600 DIY house kits from 1908 through the 1940s, resulting in an estimated 100,000 'Sears and Roebuck homes.'

The Brothers Grimm were lawyers before they were storytellers, but became interested in fairy tales as they began investigating how rules and customs had been passed down through cultures.

The Mona Lisa's celebrity status and repeated restorations have obscured what made the painting so revolutionary for its time

The distributor behind 'Rocky Horror' pulled the film from its scheduled national release, believing it would be a failure, though it eventually amassed a dedicated fanbase through midnight showings, a trend that continues over 50 years later.

Visualize the path of every solar eclipse from 1900 to 2099 and see what each looks like when observed from different locations on Earth.

Some renewable energy sources may weaken the resiliency of electric grids by removing outage buffers provided by coal- and natural gas-fired plants, which provide the system with more time to find a new power source if a plant goes down.

Domino's Pizza's 30-minute delivery guarantee was responsible for the chain's 1980s surge, but it was discontinued after the company was found liable for delivery driver accidents.

The United States is one of only around two dozen countries with a fully federal system of government, in which power is divided between national and state governments.

Browse a detailed visual analysis of over 200,000 similes—comparisons that often use 'like' or 'as'—from popular fiction to see how frequently writers the same idiomatic expressions.

NBA superstar Victor Wembanyama spent the 2025 summer training with monks in central China to enhance his body and mind.

Coral polyps reproduce via broadcast spawning, where entire reefs simultaneously release reproductive cells for about one hour, once a year.