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SleepSleep is far more than just a period of rest—it’s a complex, active process during which the brain cycles through distinct stages, each marked by unique patterns of brain activity.
While scientists still debate sleep’s true purpose, research has proven it to be essential for survival. Both humans and animals suffer severe consequences without sleep, including cognitive decline, emotional instability, and eventual death. More than a third of adults regularly experience poor sleep, leading to increased risks of heart attack, Alzheimer’s disease, and motor vehicle accidents.
During both wake and sleep states, the brain generates electrical rhythms called brainwaves, which represent oscillating patterns of brain cells (known as neurons) working together. Different brainwaves are associated with specific states of consciousness. For example, high-frequency beta waves dominate during alertness, while sleep features primarily theta and delta waves.Explore Sleep
What we've found
HRT for menopause may not help with mental health or sleep says a large 2026 analysisA recent analysis investigating the effects of menopause and HRT use on mental health, cognition, and brain health suggested HRT did little or nothing to aid with mental health and sleep. But the researchers caution that, among other data gaps, they didn't have information on which HRT formulations were used among their nearly 125,000 participants, so results may have varied by drug treatment. Smithsonian MagazineElon Musk is known to sleep under his desk during intense periods of workThe tech company founder and CEO has a wide variety of leadership principles he has kept consistent while building multiple different companies, including eschewing middle management and using the "idiot index," a formula he uses to identify manufacturing inefficiencies. Founders PodcastExplore a series of bedtime exercises to improve sleepLow-impact, mindful movements can potentially help the body and mind relax before sleeping. Such exercises could be a helpful complement to regular exercise during the day. AARP'It might be that some day I shall be drowned by the sea, or die of pneumonia from sleeping out at night, or be robbed and strangled by strangers. These things happen. Even so, I shall be ahead because of trusting the beach, the night and strangers.'-Janet Reno, the US' first female attorney general (1938-2016) International Business TimesOlympians view sleep as a performance tool, prioritizing routine and environmentElite athletes emphasize consistent sleep routines, dark and cool environments, the well-placed nap, and tools like eye masks, white noise machines, and supplements like melatonin (particularly for jet lag). This priority belies a core principle: recovery and performance are inseparable. Read more ways Olympians "sleepmaxx" here. BustleArtificial lighting and modern practices like working late are linked with sleep shiftsAn 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. Journal of Clinical Sleep MedicineSleep deprivation leads to buildup of danger molecules in the gutFruit flies genetically engineered to be awake for as long as they live tend to live half as long as their well-rested brethren. Analysis shows these modified flies suffer from a buildup of DNA-destroying reactive oxygen species in their guts. When enabled to sleep, the flies live as long as their counterparts. Quanta MagazineGeorge Pullman popularized tipping in the US with his Pullman sleeping car businessBy 1868, Pullman, who operated luxury sleeping cars on the country’s growing network of railways, had started hiring formerly enslaved Black men as porters for extremely meager wages, working long hours in poor conditions. He encouraged wealthy passengers to tip them, helping institutionalize tipping in the US. Restaurant Business OnlineThe US loses roughly $400B a year in productivity due to sleep deprivationFor 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. HubSpotSleep is often viewed individually, when its impact and meaning may be more communalAt 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. AeonSleep research indicates dogs likely dream about their toys, food, and ownersAccording to the continuity hypothesis, dreams are built from waking life. This likely applies to pets. Repetitive barking, whining, or running movements while asleep indicate a dog is dreaming. Discover MagazineObsessing over sleep metrics leads to worse sleep qualityOver one-third of Americans have used wearable technology to track their sleep, and the anxiety over having perfect sleep data, or orthosomnia, has made getting a good night's sleep more difficult. Experts agree that most devices cannot correctly assess sleep details, making the data unreliable. Popular ScienceYour brain moves through four distinct sleep stages every nightEach stage plays a key role in memory, repair, restoration, and other key biological functions. The full cycle generally proceeds from light sleep and deep rest into vivid REM dreaming, which repeats between three and five times nightly. 1440High-quality sleep is effectively a superpowerSleep is critical to maintaining healthy biological functions. From restoring energy to consolidating memories, aiding immune system functions and balancing hormone production, optimizing sleep habits are one of the most important ingredients to healthy living. TEDCerebrospinal fluid flushes waste from your brain during sleepRecent research suggests a nightly brain cleanup as an important purpose of sleep. Brain waves have been observed to work in lockstep with the brain’s blood flow and cerebrospinal fluid flow, allowing for a cleansing tide that washes away toxic waste. Scientific AmericanDo sleep-tracking apps work?Can technology help us sleep better? As wearables and apps promise to optimize our nights, this New York Times investigation reveals some troubling findings after columnist Brian Chen used a wearable for two weeks to track his sleep. He discovered that sleep trackers not only provide frequently questionable data but can also paradoxically worsen sleep problems by causing users to obsess over their metrics — a new condition researchers are calling ‘orthosomnia.’ The New York TimesA sleep expert discusses the broad benefits of quality restIn this three-part series, renowned sleep expert Matthew Walker and doctor Peter Attia explore the role of sleep in memory consolidation and mental health, and the detrimental effects of chronic lack of sleep. The latter has been linked to increased risk of dementia, hypertension, and more. Peter AttiaThe brain consolidates memories during sleep During sleep, the brain appears to process memories from the day and encode them into long-term storage. At the same time, the brain prunes old memories out of the brain that are no longer useful. Understanding this process also sheds light a various neurodegenerative diseases. Yale UniversityA deeper walk through of the four stages of sleepIn this TED video, sleep expert Matthew Walker carefully breaks down the sleep stages along with a description of why each is important. He finishes the discussion by examining the consequences of altered or decreased sleep, and why seemingly small alterations in our sleep patterns can have outsized effects. TEDForty to 110 minutes of deep sleep is recommended per nightAdults 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. Sleep FoundationUnderstanding the stages of sleepDo you have trouble keeping the different stages of sleep straight and organized in your mind? Us too. Sometimes it helps to have a straightforward listing of what each stage is and why it’s important. Look no further than this resource from Lumen. It is a basic outline of the four stages of the sleep cycle, along with the brainwaves and attributes that characterize each. If you’re brave enough, test yourself on the built-in quiz! Lumen LearningWhat happens when we sleepSleep 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. The EconomistDelta brain waves happen during deep sleep, while REM cycles resemble being awakeAs the brain cycles from wakeful rest to deep sleep, different brain waves emerge. Alpha waves occur during restful waking states, while theta waves can be associated with light sleep and meditation. Delta waves are observed during deep sleep—but REM cycles more closely resemble active awake states. The ConversationSimilarities between human and animal sleepHave you ever noticed the wide-ranging differences in sleep between humans and animals? Sleep as a necessity is present amongst all animal species, but the specific patterns of sleep vary widely. Just think of the differences between how an owl, a whale, and a horse might sleep. This helpful resource from the Sleep Foundation explores the relations between human and animal sleep, complete with some fascinating statistics about sleep patterns throughout the animal kingdom! Sleep FoundationHumans may have evolved to need less sleepHumans 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. 1440Dangers of a record-breaking, sleep-skipping stuntIn 1963, 17-year-old Randy Gardner decided to stay awake for as long as it took to break the current world record. He managed to stay awake for 11 days. In 2017, the then-71-year-old Gardner told NPR that his experience had come back to haunt him as insomnia. The challenge has since been removed from the Guinness Book of World Records due to its dangerous consequences. NPRSome animals seem to live well on little sleepAfrican 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. Oxford University PressWhat sleep loss does to the brainProteins in the brain perform various critical functions, such as helping neurotransmitters function or enabling electrical activity in the organ. These proteins must fold in specific ways to perform their duties, similar to paper airplanes. Sleep deprivation contributes to the gathering of misfolded proteins that aren't shaped correctly for their roles and so accumulate in undesired spaces, such as in the gaps between neurons. Oxford UniversityThe brain will attempt to rest itself in periods of chronic sleep deprivationSkimping on sleep lengthens reaction times and blunts decision-making skills. At a certain point, the brain will try to sneak sleep where it can, shifting sections offline even as you remain awake. Going without sleep raises blood pressure, undermines the immune system, and increases one's risk of dementia. Science VsSleep disorders can exacerbate neurological and other health conditionsBrain regions that control sleep sit adjacent to those that regulate the autonomic nervous system, which controls involuntary bodily functions like heart and breathing rates. These neighbors influence each other, meaning poor sleep can impair or negatively impact the ANS, further disturbing sleep. National Institutes of HealthClinical insomnia is defined as trouble sleeping for three nights per week for at least one monthThe term "insomnia" is often used in popular culture to refer to general difficulty sleeping or staying asleep. In the US, insomnia transitions to a clinical disorder when symptoms—dissatisfying, nonrestful, and life-disrupting sleep—occur at least three nights per week for at least one month and cause "clinically significant" distress in important areas of life. PsychDBStudies suggest when you sleep is as important as how much you get"Molecular clocks" made of timekeeping proteins and genes help regulate the body and its processes, meaning the disruption of one cog will impact another. Those who live in sync with their circadian rhythms report less fatigue, clearer thoughts, greater medication benefits, and better long-term health outcomes. ReutersSleep paralysis occurs when the body fails to 'unfreeze' in sync with wakingWhen the body falls into a deep enough phase of sleep, it paralyzes certain parts of itself so the brain can't force it to physically act out dreams. This well-known but little-understood phenomenon happens when the brain awakens early and typically results in haunting hallucinations and reports of supernatural terrors. VoxDreams largely occur during REM sleep (but not entirely)Modern imaging tools have enabled scientists to carry out in-depth studies of brain activity during sleep. Researchers know dreams arise largely during REM, blending emotion-driven visual imagery, memory consolidation, and weak associations to help us make sense of experiences, regulate emotions, and prepare for future situations. Quanta MagazinePoor sleep increases the risk of developing Alzheimer'sBrain imaging studies reveal poor sleep leads to production of amyloid beta proteins—molecules linked to Alzheimer’s—which are cleaned from the brain during high-quality sleep. These findings suggest conditions like sleep apnea and insomnia increase the risk of developing the disease. TED ConferencesThe body's sleep-wake cycle is one of its many circadian rhythmsThe body's sleep-wake cycle is one of its many circadian rhythms, which are behaviors or biological processes that exhibit 24-hour patterns. These rhythms are deeply impacted by light exposure, food availability, and much more, meaning where you live impacts your sleep. alie wardThe evidence for the best sleep positionsFor a behavior so crucial to human health, sleep positions are reportedly understudied by researchers. This article summarizes some of the more recent studies, taking into account the various forms of problems encountered during sleep: neck and back pain, snoring, acid reflux, and facial skin damage. Most studies point to side-sleeping as the most optimal, with sleeping on one's front as the least helpful. Read the piece to see which position is best for you. BBC NewsThe lost medieval habit of 'two sleeps'While most people accept a single seven-to-nine-hour nightly slumber is ideal, the approach is a relatively modern idea. For centuries, interrupted sleep was the norm, with historians uncovering descriptions of two periods of sleep during the normal 24-hour cycle. BBC NewsA 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. The MessengerSleep deprivation may slow metabolismThis article explains that, according to research from Penn State, restricting sleep for just a few days can affect how the human body metabolizes fats and change how satisfied people feel after a meal. The study found that people who don’t get... Penn StateGibberish, urine, and utter chaos: what happens when you sleepwalkSleepwalking, or somnambulism, occurs most frequently in children who are in the deepest stages of sleep. REM, the sleep stage in which most people dream, often causes temporary muscle paralysis, so it’s only when sleepers pass to the deeper stages of sleep that sleepwalking—and sleep talking—can occur. TED-EdSleep apnea explainedWhile sleep apnea is directly tied to an increased risk of cardiovascular and metabolic health problems, many affected people are unaware of their problem. Sleep apnea occurs when upper airway muscles relax during sleep and pinch off the airway, which can cause breathing to pause for 10 seconds or more at a time. Sleep partners (rather than those inflicted) are often the first to notice breathing pauses or loud snoring in their bedmates. National Institutes for HealthExternal factors, not aging itself, lead to sleep loss in older adultsThough it is often believed that people need less sleep as they age, the recommended sleep guidelines for all adult age groups are almost identical. However, factors such as medication use, lack of physical activity, and loneliness can lead to sleep problems, and thereby lead older people to believe that they actually need less sleep than they do. External factors, not aging itself, lead to sleep loss in older adultsNarcolepsy occurs when the brain loses its ability to control its sleep-wake cycleNarcolepsy not only makes people fall asleep at random moments, but it can also cause hallucinations, insomnia, and sleep paralysis. The disorder is tied to cells in the brain’s hypothalamus called hypocretin, which are neuropeptides responsible for keeping you awake and controlling your dreaming from coming on too quickly. SeekerHow much sleep do you need?When it comes to how much sleep you need, age matters—and, for your health, it's important to be well rested. Young children between the ages of one and three need the most sleep: It’s recommended that toddlers get between 11 and 14 hours of... CNNSleep training: risks and benefitsSleep training, which can range from implementing a nighttime routine to letting your baby cry it out, is controversial, but does it work? While dozens of studies have found sleep interventions to be effective, it appears that many of these studies... BBC NewsHotels around the world now offer packages focused especially on sleepA Belmond Hotel in London, for example, has partnered with a meditation and holistic coach to create a service called the Sleep Concierge, which includes meditation recordings, a pillow menu, and the option of a weighted blanket, among other customized options. Medical experts believe that hotel experiences such as this one can be beneficial to those lacking sleep, provided certain provisions (such as an on-call medical professional) are also provided. CNNWhat sleep is for and why you can’t live without itSleep is one of the most important needs that humans have. The body uses sleep as a time of restoration and rejuvenation, as body tissues repair themselves, growth hormones are released, and the process continues. Brain plasticity theorists believe that sleep is also used for the processing and consolidating of new memories. SciShowSAD disrupts melatonin secretion, producing it longer in the morningThe hormone is secreted from the pineal gland in the brain and is key to the timing of sleep. However, the substance does not generate sleep itself. With seasonal affective disorder, the body misperceives the photoperiod—the time with light—and the cycle of sleeping and waking gets off-phase, leading to issues with circadian rhythms. TEDThe REM cycle happens around four to six times per nightSleep is a natural process that is critical to maintaining life—a process conserved throughout evolution across just about any animal with a brain. Generally speaking, the first three stages involve increasingly deep phases of non-REM sleep, followed by a roughly 90-minute period of REM sleep that repeats throughout the night. National Institutes of Health
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