Sleep is something we do every day– in fact, we spend a third of our entire lives asleep– and yet to many of us, it largely remains a mystery. At Sleep Advisor, we think it’s important to understand sleep – what is happening physically and mentally while we’re sleeping, the science of sleep, various sleep disorders, environmental factors that affect our sleep, and much more– so that we can better understand this experience, and ultimately, get the best quality sleep possible.
That’s why we’ve put together this extensive glossary of terms relating to sleep. We hope that learning more about this terminology helps provide you with more context about sleep. This list includes 100+ key words and phrases, many of which you are probably familiar with and some of which might be new to you. Ultimately, we hope this sleep terms glossary will help deepen your conversations and understanding of sleep.
Actigraphy1 is a technique used to measure cycles of activity and rest over several days or weeks. It’s usually measured by wearing an actigraph, a small wristband-like device, which measures sleep cycles and can help diagnose certain sleep disorders.
When we use the word “acute” in terms of sleep, we’re talking about a condition or state that is not permanent but rather short-lived. For example, acute sleep deprivation2 usually lasts for a matter of days, whereas chronic sleep deprivation is longer-term.
Artificial light includes any source of light produced by electricity or artificial means. In terms of sleep, artificial light can have an impact on your circadian rhythm.
Atonia3 means loss of muscle definition. During REM sleep, it is normal to experience atonia. Conversely, some sleep disorders are caused by the lack of atonia during REM sleep, which can lead to excessive muscle twitching or movement.
Attention-Deficit/Hyperactivity Disorder (ADHD)
ADHD4 is a common neurodevelopmental disorder, in which symptoms include difficulty paying attention, inability to control impulsive behaviors, or over-activity. It is typically diagnosed in childhood and often lasts into adulthood.
Awakenings5 are the moments we wake up throughout our sleep at night. Usually, mini-awakenings go completely unnoticed and can occur up to 20 times per hour. When you become aware of an awakening, it becomes an observable wake-up. This is also normal and can occur about two or three times per night.
Bilevel Positive Airway Pressure (BPAP) Device
A BPAP is a type of ventilator – a device that helps with breathing. This machine helps push air into the lungs via a mask and nasal plugs that are connected to the ventilator. It is often prescribed for people with sleep apnea, COPD, or other conditions that affect breathing during sleep.
Biphasic sleep is sleep that is divided into two sessions per day. Rather than sleeping once throughout the night, a biphasic sleeper might sleep for four hours at night and four more during the day. This was a practice much more common in medieval times6 and these days is more often caused by a sleep disorder.
Blue light7 is one of the colors of light on the electromagnetic spectrum. Like the other colors on the spectrum, blue light is naturally emitted by the sun; but more often, when we refer to blue light we are talking about the blue light from screen devices like cell phones and computers. While this light can be beneficial during the day, as it boosts attention, alertness, and mood, it can have a negative impact on sleep when viewed at night.
Brown noise8 is a low-frequency noise that can help listeners feel relaxed. Where white noise contains all the frequencies of sound blended equally, brown noise puts more energy on the lower frequencies, creating a more bass type of sound. It is thought to be useful in helping people focus, though more research needs to be done on this.
Cataplexy9 is one of the physical features of the sleep disorder narcolepsy. It includes episodes of voluntary muscle weakness followed by intense emotion.
Caffeine is a commonly used mild stimulant that is most often found in coffee, sodas, teas, dark chocolate, certain supplements, and energy drinks.
Central Sleep Apnea
Also called CSA, Central Sleep Apnea10 is when the brain doesn’t send the proper signals to the muscles that control breathing, resulting in repetitive periods of breathing disturbances while sleeping. CSA is a sleep disorder that can lead to an increased risk of cardiovascular problems.
This is your body’s natural inclination to sleep or be awake at certain times. For example, morning people are a certain chronotype, and night owls are another. Chronotypes also affect when you’ll want to eat, exercise, and your core body temperature.
Circadian rhythm is the biological tendency to operate in 24-hour cycles of sleeping and waking. Circadian rhythms are present in all organisms – animals, plants, and even fungi and bacteria.
Circadian Rhythm Sleep Disorder
Circadian rhythm sleep disorders11 are also known as sleep-wake cycle disorders and are problems that occur when your internal clock is out of sync with your environment. This results in difficulty sleeping and poor sleep quality.
Cognitive function describes the use of one’s brain. It includes the mental processes of perception, learning, memory, understanding, reasoning, awareness, intuition, and language.
This occurs when a person’s cognitive function is impaired. It can affect only one process – as Alzheimer’s does with memory – or it can affect multiple processes, like memory, learning, and reasoning.
Cognitive Behavioral Therapy
Cognitive Behavioral Therapy12, or CBT, is a type of therapy commonly used to help train the brain to create new thought patterns and emotions. CBT has become a popular tool for treating insomnia and includes intentionally changing thoughts about sleep, as well as improving sleep hygiene.
CPAP (Continuous Positive Airway Pressure)
A CPAP is a device that attaches to the nose and delivers a continuous stream of air while the wearer sleeps. This is commonly prescribed for those with obstructive sleep apnea.
Also called daytime somnolence13, daytime impairment is the inability to stay awake and alert during the normal waking periods of the day. This results in drowsiness or unintentional lapses into sleep, and can lead to vehicle accidents, loss of productivity, and cognitive impairment.
Deep sleep, or slow wave sleep, is the most restorative sleep stage. It is the third stage of the sleep cycle and is the stage in which the body repairs itself. It is categorized by the lowest amount of brain activity and is vital for proper functioning. It is also hardest to wake up from deep sleep.
Delayed Sleep Phase Syndrome (DSPS)
This circadian rhythm sleep disorder is characterized by the inability to fall asleep at a “normal hour,” or beyond two hours of what is socially typical. It also results in the sleeper naturally waking up later than is considered normal. The term “night owl” is often used to describe these sorts of sleepers.
Delta sleep14 is another term for deep sleep or slow wave sleep. During deep sleep, the brain is emitting very low frequency, high-amplitude fluctuations called delta waves. These slow waves make it difficult to wake up during this stage of sleep.
Dementia is the general term for the impaired ability to remember, think, or make decisions. Alzheimer’s disease is the most common type of dementia and can have a major impact on sleep, and vice versa.
This refers to something that takes place during the day, or daily. Diurnal sleep refers to sleeping during the day, rather than at night.
Dreams are images, sounds, and sensations that we experience while we’re asleep. They can occur at any time throughout the night, but they are most common (and memorable) during REM sleep.
Learn more: Why Do People Dream At Night?
This term describes the ability to remember your dreams. The ability to recall your dreams can depend on various factors15, such as your personality, mental state, creativity, cognitive function, quality of sleep, mental health, etc.
Epworth Sleepiness Scale
The Epworth sleepiness scale16 is an evaluation used to determine your level of daytime sleepiness.
An electroencephalograph (EEG)17 is a test that measures activity in the brain. It is administered with small, metal electrodes attached to the scalp, which show up as wavy lines on an EEG recording. In sleep, an EEG can be used to determine somebody’s quality of sleep.
Entrainment is the synchronization18 of your biological clock to external cues, like the sun rising and setting.
Excessive Daytime Sleepiness (EDS)
Also called hypersomnia, excessive daytime sleepiness (EDS) is a sleep disorder characterized by the inability to stay awake during the day. It is different from feeling sleepy or fatigued throughout the day, in that with EDS, you’re physically unable to stay awake and may fall asleep at inappropriate moments.
Fatigue is when you feel lethargic or sleepy. Fatigue affects the body, mind, or both, and it is common among people with depression or sleep deprivation.
Generalized Anxiety Disorder
Generalized anxiety disorder (GAD)19 is a mental health disorder characterized by persistent feelings of anxiety or dread that interfere with daily life. It is different from occasional worrying, stress, or anxiety, in that it is not related to a specific situation and can last months or years.
Hormones are chemicals that carry messages throughout the body to organs, skin, muscles, and other tissues. They communicate different functions and are essential for life, health, and sleep.
A constant state of physical and mental tension during the day and into the night. It can be the result of PTSD or stress and puts the brain in a state of hyper-alertness. This constant alertness can have a negative impact on sleep and cause hyperarousal insomnia20.
Another term for excessive daytime sleepiness, hypersomnolence can also include sleeping longer than usual at night.
Hypersomnia is when you regularly experience hypersomnolence.
Hypnagogic sleep is the state immediately before falling asleep as we transition from wakefulness to sleep. It can include hypnic jerking, which is an involuntary twitch that occurs just before falling asleep.
During hypnagogic sleep, we can experience brief hallucinations21. They’re usually more visual and include patterns, shapes, and flashing lights. These are considered normal, though.
A hypnogram is a graph22 that represents the stages of sleep over time.
Hypnopompic hallucinations23 are similar to hypnagogic hallucinations. Rather than occurring when a person is going to sleep, though, these hallucinations can happen when a person is waking up. Similarly, they are visual and can consist of moving shapes, colors, and images, and sometimes include sound.
In terms of sleep, the term “hypnotic” can refer to two different things: The state of being hypnotized with hypnotherapy at bedtime to sleep better, or a class of drugs called sedative-hypnotics24 prescribed as sleep aids. These can include drugs like Ambien.
Hypoxia means you have low levels of oxygen in your blood tissue. This condition can result in confusion, restlessness, difficulty breathing, bluish skin, or rapid heart rate. When hypoxia occurs during sleep it is called sleep-related hypoxemia disorder.
Insomnia is a sleep disorder in which the individual has trouble falling or staying asleep. It can be caused by various factors like stress, illness, irregular sleep schedules, mental health, lifestyle, medications, neurological issues, or other sleep disorders. Insomnia is one of the most common sleep disorders.
Jet lag is a short-term sleep disorder caused by traveling across multiple time zones by airplane. This change in time zones can interfere with your usual circadian rhythm and make it difficult to fall asleep at a normal hour in your new destination. Jet lag symptoms include fatigue, stomach issues, feeling “off,” impaired cognition, and bad mood.
Light sleep describes the first two stages of the sleep cycle, just before the third stage, deep sleep. These stages are referred to as light sleep because it’s easier to wake up during this time.
Also called bright light therapy25 , light therapy is used to treat various ailments such as insomnia, circadian rhythm sleep disorder, jet lag, and seasonal affective disorder. This is done by exposing a person to artificial light for a certain amount of time each day, usually from a visor or lightbox.
Long Sleep (Type of Hypersomnia)
Long sleep is a type of hypersomnia26 in which you sleep longer than usual at night. This means sleeping for more than 9 or 10 hours per night.
A lucid dream means you’re aware that you’re dreaming. You can train yourself to lucid dream with practice.
Maintenance of Wakefulness Test (MWT)
The maintenance of wakefulness test (MWT) is a polysomnographic procedure27 that measures your ability to remain awake during normal waking hours in a sleepy environment like a dark, quiet space. The test can help sleep specialists identify the severity of symptoms of disorders like narcolepsy.
Melatonin is a hormone naturally produced by the body that helps create a feeling of sleepiness. It is produced in response to darkness and can be adversely impacted when exposed to artificial lights late into the night. Melatonin can also be taken as a supplement to treat insomnia, jet lag, and other sleep-related issues.
Metabolism is the process by which the body converts food and drinks into energy, which then fuels functions like breathing, blood circulation, digestion, cell growth and repair, managing hormone levels, and body temperature regulation.
Microsleeps are sleep episodes that last only between three and five seconds, generally with the eyes still open. They’re often the result of being sleep deprived and are dangerous since they severely impact cognitive performance, mood, and motor function.
Learn more: Microsleeps and Micro Naps
Mixed Sleep Apnea
Mixed sleep apnea28 , also known as complex sleep apnea, is when someone has both obstructive sleep apnea and central sleep apnea.
Monophasic sleep is when someone gets all of their sleep at night, rather than having their rest broken up into nighttime sleeping and regular daytime sleeping.
Multiple Sleep Latency Test (MSLT)
The multiple sleep latency test (MSLT)29 is used to measure the extent of a person’s daytime sleepiness, as well as how quickly REM sleep begins. In the test, participants are given four to five opportunities to sleep every two hours during normal waking hours.
A nap is a brief period of sleep during the day. There are many different types of naps, and some may work better for you than others.
Narcolepsy is a sleep disorder in which the brain can’t control the body’s ability to sleep or stay awake. The most pronounced symptom of narcolepsy disorder30 is falling asleep during the day, often at inopportune times.
Natural light is light that is generated by nature. The most common source of natural light is the sun, but other sources include other stars, the moon, lightning, or fire. Natural light impacts your body’s natural circadian rhythm.
Neurotransmitters are the body’s chemical messengers that carry signals from a neuron to another target cell, which can be either a nerve cell, muscle cell, or gland. Some of the more commonly known neurotransmitters31 include serotonin, dopamine, norepinephrine, endorphins, and GABA.
Night sweats are repeated episodes of heavy sweating during sleep. Night sweats can be caused by stress, anxiety, obesity, idiopathic hyperhidrosis, certain medications, hormonal fluctuations (like during menopause), and certain illnesses.
A night terror32 is a sleep disorder in which a person suddenly wakes up in a state of terror. Other symptoms include screaming, sweating, confusion, rapid heart rate, no recall of bad dreams, and inability to fully wake up. Unlike nightmares, night terrors occur during NREM sleep.
A nightmare is a dream with scary, troublesome, or disturbing themes that usually occurs during REM sleep in the second half of the night33 .
A nightmare disorder is when someone experiences frightening and disturbing dreams night after night, which can cause stress and negatively impact the individual’s life.
Nocturia is waking up in the middle of the night to urinate. Drinking too much before bed, certain sleep disorders, and bladder obstruction are examples of reasons why someone may experience this.
Nocturnal refers to anything that occurs during the night. This term can also refer to animals who are more active during the night. Owls, bats, and raccoons, for example, are nocturnal animals.
Non-24-Hour Sleep-Wake Disorder (N24)
This sleep disorder is when a person’s biological clock doesn’t synchronize with the 24-hour day. Instead, someone with N2434 will find themselves staying up later and later each night and sleeping in later and later each morning. It is most common among the blind community.
Non-Rapid Eye Movement (NREM) Sleep
The first three stages of sleep are non-rapid eye movement (NREM) sleep. In these three stages, the eyes remain still and the breathing, heart rate, and brain activity all slow down, the body temperature drops, and the muscles relax.
Learn more: REM and NREM Sleep Stages
Obstructive Sleep Apnea (OSA)
Obstructive sleep apnea is the most common type of sleep apnea, which is caused by an obstruction blocking your airways. This obstruction is often the relaxed muscles in the back of the throat. OSA leads to low oxygen levels in the body, which causes people to wake up abruptly during the night.
A parasomnia is any type of abnormal nighttime behavior, such as sleepwalking, talking in your sleep, bedwetting, nightmares, or night terrors.
Want to learn more about parasomnias? Visit our in-depth guide to parasomnias.
Partial Sleep Deprivation
Partial sleep deprivation is when you sleep for less than six hours35 at night.
Periodic Limb Movement Disorder (PLMD)
Periodic limb movement disorder is when you experience repetitive limb movements36 that disrupt or disturb sleep.
Pink noise is used to drown out background noise to promote relaxation. Like white noise, pink noise37 plays all frequencies of sound, but the higher frequencies are diminished and lower frequencies are emphasized.
A polysomnography is a type of sleep study38 that records certain body functions as you sleep in order to diagnose sleep disorders.
Power naps are short daytime naps that last between 10 and 30 minutes. They’re meant to give you an energy boost if you’re feeling sluggish.
Discover more: Power Nap Benefits
Progressive Muscle Relaxation (PMR)
This is a technique to relax the body and mind, and it’s often used to help people fall asleep. In PMR, you focus on muscles throughout the body, usually starting with the toes and moving up, and progressively focusing on relaxing them until your entire body is relaxed.
Rapid Eye Movement (REM) sleep
This is the fourth and final stage of sleep, following the three NREM stages. In REM sleep, the eyes move rapidly and brain activity increases. This is also the sleep stage when dreaming typically occurs.
Restless Legs Syndrome (RLS)
RLS is a neurological disorder39 that causes an irresistible urge to move the legs. It is most common in the later afternoon and evening hours and gets worse at night for most people, which can impact sleep quality.
Discover our top picks for the best mattress for restless leg syndrome.
Screen time represents time spent looking at a screen on devices like cell phones, computers, televisions, and tablets. These screen devices also emit a blue light that can be disruptive to sleep when used at night.
Find out more: Effect of Lights on Sleep Quality
Seasonal Affective Disorder (SAD)
Also referred to as seasonal depression, seasonal affective disorder is a type of recurring depression that occurs with the changing of the seasons. For most people, this occurs when the temperatures drop and the hours of daylight decrease. It can impact mood, daily life, and sleep.
A sedative is a class of drug40 prescribed to slow brain activity and induce a state of calmness or sleepiness. Examples include benzodiazepines, barbiturates, and hypnotics.
Shift Work Sleep Disorder
Shift work sleep disorder usually affects those who work non-traditional hours. For example, people who work overnight may have difficulty falling asleep, staying asleep, and sleeping during the day.
Learn more about how shift work affects the circadian rhythm
Short sleep means either sleeping for a short period of time (like a nap) or naturally and regularly sleeping for less than the recommended amount of sleep per night and still feeling rested.
A sleep aid is a substance taken before bedtime to help people fall asleep and sleep better. This could be a prescription medication, like Ambien, an over-the-counter medication, or a naturally-occurring substance like chamomile, magnesium, or melatonin.
Sleep apnea is a sleep disorder in which your breathing repeatedly starts and stops throughout the night, causing low oxygen levels that cause you to wake up frequently. The three types of sleep apnea are obstructive, central, and complex/mixed.
Check out our picks for the best mattress for sleep apnea.
This term refers to the basic structure of normal sleep, including the three stages of NREM sleep followed by one stage of REM sleep.
A sleep clinic is a facility where professional sleep studies are held.
A sleep cycle includes one complete loop through the three stages of NREM sleep and the one stage of REM sleep. A full sleep cycle takes about 90 to 110 minutes, and a normal night’s sleep should include about four to six complete cycles.
Sleep debt is the difference between how much sleep a person needs and how much sleep they’re actually getting. For example, a person who needs eight hours of sleep and only gets six hours for three nights will have a sleep debt of two hours per night or six hours cumulatively.
Sleep deprivation occurs when someone doesn’t get the recommended amount of sleep for their age group. It can be acute, meaning a lack of sleep for a short period of time like a few days, or it can be chronic, which is a lack of sleep for three months or longer.
A sleep disturbance is any sleep disorder that involves difficulty going to sleep or maintaining sleep, excessive sleepiness or somnolence, sleep-wake schedule disorders, or dysfunctions associated with sleep, sleep stages, or parasomnias.
Sleep disorders are repeated problems with the quality, timing, or amount of sleep you’re getting, and these disorders can negatively impact daily life. There are 80 different types of sleep disorders, some of the most common being insomnia, sleep apnea, and RLS.
Sleep duration is the amount of time a person spends sleeping, measured over the course of a 24-hour day or in one sleep period.
Sleep efficiency is the amount of time during a sleep period that is actually spent asleep. This is calculated41 by dividing the total time spent asleep by the total time spent in bed.
Sleep environment is the space where you sleep, which is usually a bedroom. It is important to create a sleep environment that is conducive to good sleep, including keeping the temperature cool, keeping the room dark, eliminating noise, and using a quality mattress.
Learn more: What is the Ideal Sleep Environment?
Sleep hygiene is a general term for any daytime and nighttime habits that impact your sleep. Good sleep hygiene would involve intentionally creating a sleep environment and practicing daily habits that support you in getting a good, uninterrupted night’s sleep.
Sleep inertia is a temporary grogginess42 or disorientation after waking up from sleep. It can decrease cognitive function, memory, reasoning, and mood, and it usually doesn’t last longer than 30 minutes.
Sleep latency is the time it takes a person to fall asleep once the lights are out and they go to bed.
Sleep medicine is a medical specialty focused on diagnosing, studying, and treating sleep disorders and other sleep issues.
Sleep meditation is usually done before bed and is geared toward winding down the body and mind in order to get a better night’s sleep.
Sleep onset is the transition from being awake to being asleep.
Sleep paralysis is the temporary inability to move, usually just after falling asleep or waking up. This is a normal part of REM sleep. However, it can feel terrifying when the person feels awake and aware of the paralysis but they remain unable to move.
Sleep pattern refers to a person’s schedule of going to bed and waking up. Sleep pattern also includes regular nap behavior, if applicable.
Sleep phase is the timing of a person’s sleep period. If a person goes to bed early and wakes up early, this is an advanced sleep phase. If a person goes to bed late and wakes up late, this is a delayed sleep phase.
Sleep quality represents how well a person feels they slept during the night. This is often determined by how refreshed and rested they feel in the morning.
Sleep regression is usually used to describe when an infant who was sleeping well suddenly doesn’t sleep well. This might include multiple wake-ups through the night and more crying and fighting at bedtime. They often coincide with developmental milestones.
Sleep spindles are bursts of brain activity43 visible on an EEG. They tend to be most evident during stage two of NREM sleep.
This is the body’s natural rhythm of being awake and being asleep during a 24-hour period. The sleep-wake cycle is tied to how light or dark it is outside.
Sleepwalking is a parasomnia that involves walking during NREM sleep when muscles are more relaxed and breathing is slowed. Sleepwalking is more common in children and teens than in adults.
Sleeping pills are prescription or over-the-counter medications used to treat the symptoms of insomnia or other sleep disorders and help the person get to sleep. They include hypnotics, sedatives, sleep aids, and tranquilizers.
This is the position somebody lies in while sleeping. The most common sleeping positions are on the back, stomach, side, or some combination of these positions.
Learn more: The Best Sleeping Positions
Slow-wave sleep is another name for deep sleep or delta sleep. During slow-wave sleep, the brain emits slow, low-frequency fluctuations.
Somnolence is when you feel sleepy or drowsy.
Snoring is a sound that comes from the throat and/or nose of a sleeper whose airways are partially blocked. Snoring can be caused by alcohol consumption, obesity, nasal obstruction, a deviated septum, enlarged tonsils, a small lower jaw, or sleeping on your back.
Discover our picks for the best mattress for snoring.
This is a term for inducing sleepiness.
Stage 1 Sleep
This is the first stage of sleep, immediately after falling asleep. This Non-REM stage of sleep usually lasts for about five to 10 minutes.
Stage 2 Sleep
This is the second stage of Non-REM sleep, which usually lasts for about 20 minutes. The heart rate and breathing slow down and muscles relax during this stage.
Stage 3 Sleep
This final stage of Non-REM sleep and is the deepest sleep stage. During this stage, breathing and heart rate are at their slowest, the muscles are the most relaxed, memories are consolidated, and the body heals itself. This stage, also called “deep sleep,” “slow-wave sleep,” or “delta sleep,” usually lasts for about 70 to 90 minutes.
Stage 4 Sleep
This is the final stage in the sleep cycle before the entire cycle repeats itself again. Stage 4 is REM sleep. Dreams typically occur during this time. The body is also paralyzed, but the mind becomes more active.
Suprachiasmatic Nucleus (SCN)
Located in the hypothalamus, the suprachiasmatic nucleus44 is made up of 20,000 nerve cells. This master clock receives information from our eyes, like when it is light outside and when it is dark, and tells our body when to feel tired (among many other functions).
Thermoregulation is the body’s ability to maintain its internal temperature, independent of the external temperature.
Learn more in our in-depth guide on thermoregulation
Time in Bed
This is the total amount of time somebody spends in bed at night, regardless of whether or not they are sleeping.
Total Sleep Time (TST)
This refers to the total amount of time that a person actually spends asleep during a sleep period, regardless of how much time they spend in bed.
A vivid dream is one that feels especially realistic or clear.
White noise is a sound, often emitted from a white noise machine, that combines all audible frequencies of sound played at the same amplitude. This static-like sound can be used for blocking out disruptive background noise to fall asleep or focus.
A zeitgeber is a natural or human-made environmental time cue45 that helps entrain somebody’s circadian rhythm to a natural, 24-hour cycle. The sun, for example, is a powerful, natural zeitgeber. Alarm clocks are another example of zeitgebers.
Sleep is a vital part of our lives; in fact, we literally couldn’t live without it. We hope to educate and answer all of your sleep-related questions here, from how to avoid nightmares to the best bedtime routines and everything in between.
One thing you can do to improve your sleep today, though, is to make sure you’re sleeping on the best mattress for your unique body. No matter how much you know about getting proper sleep, if you’re sleeping on a mattress that causes you discomfort, it’s going to be difficult to get a good night’s sleep.
Check out our picks for the best mattresses of 2023
Natalie is a content writer for Sleep Advisor with a deep passion for all things health and a fascination with the mysterious activity that is sleep. Outside of writing about sleep, she is a bestselling author, improviser, and creative writing teacher based out of Austin.
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