National Sleep Foundation

Glossary

A

Advanced Phase Sleep Type Disorder (ASPD) - A Circadian Rhythm Disorder characterized by characterized by habitually early sleep-onset and wake times, compared with societal or preferred norms.

Alpha waves - Occur during wakefulness and periods of relaxation (i.e., during meditation). These waves are slower, and have less amplitude and variability than beta waves.

Apnea - Cessation of airflow for a minimum of 10 seconds in adults, or 2 respiratory cycle lengths in children.

Apnea-Hypopnea Index (AHI) - The average number of apneas and hypopneas obtained per hour of sleep. The index is used to diagnose sleep disorders, and is similar to the Respiratory Disturbance Index (RDI), but does not count Respiratory Effort Related Arousals (RERA).

Ascending Reticular Activating System (ARAS) - Wakefulness is promoted by the activation of the ARAS the by monoaminergic and cholinergic projections from the brainstem, thalamus, hypothalamus, basal forebrain, and cortex.

Attention-deficit hyperactivity disorder (ADHD) - A brain disorder characterized by a pattern of inattention and/or hyperactivity-impulsivity, which interferes with an individual’s ability to function, quality of life, and/or development.

Augmentation (as in restless legs syndrome) - Shifting of symptoms to an earlier time in the evening than was previously typical, the intensification of symptoms, and/or the presence of symptoms in additional body parts as a consequence dopaminergic drug therapy.

B

Bedwetting (sleep enuresis) - A parasomnia defined as recurrent, involuntary voiding of urine during sleep. Sleep enuresis is diagnosed as a disorder if it occurs chronically (i.e., at least twice per week) and does not respond to conventional therapies.

Behaviorally-Induced Insufficient Sleep Syndrome (ISS) - A hypersomnia involving chronic, voluntary sleep restriction, resulting in Excessive Daytime Sleepiness (EDS) and fatigue. Common causes include working long hours, shift work, and engaging in other activities (i.e., TV, device use) instead of sleeping.

Beta waves - Occur during daily wakefulness and have the highest frequency and the lowest amplitude, compared to other waves. These patterns also show a lot of variability.

Bilevel Positive Airway Pressure (BiPAP) - A refinement of Continuous Positive Airway Pressure (CPAP), BiPAP permits independent adjustment of the air pressures delivered during inspiration and expiration. The equipment can sense when the patient tries to exhale and drop its pressure accordingly.

Bruxism - A sleep disorder characterized by grinding and/or clenching the teeth during sleep, usually also associated with sleep arousals.

C

Cataplexy - A sudden, temporary loss of muscle tone, typically triggered by a strong emotion such as surprise, laughter, or anger. Mild cases may involve only a temporary slurring of words, buckling at the knees, or the need to sit down; severe cases might result in complete postural collapse. Cataplectic episodes last from a few seconds to a few minutes and then resolve completely.

Catathrenia (Sleep-related Groaning) - A parasomnia characterized by groaning or moaning sounds, which are usually clustered and occur within a long period of expiration (2-50 seconds) subsequent to a deep inspiration.

Central Sleep Apnea (CSA) - Transient cessation of breathing (less than 10 seconds) during sleep caused by a disturbance in the brain's respiratory centers or in the respiratory feedback to these centers. With central sleep apnea (unlike with obstructive sleep apnea), there is no effort to breathe during the period of airflow cessation.

Cheyne Stokes breathing - A breathing rhythm with a specified crescendo and descrescendo change in breathing amplitude.

Circadian Rhythm - Typically emerges two to six months after birth and is entrained by light to Earth’s daily rotation (i.e., adjusted to a 24-hour cycle). Along with homeostatic influences, the circadian rhythm helps regulate sleep and wakefulness over a 24-hour period.

Circadian Rhythm Disorders - Disorders that involve problems with an individual’s internal clock that disrupt his or her sleep patterns. Shift work disorder (SWD), is a form of Circadian Rhythm Disorder.

Cognitive Behavior Therapy (CBT) - A type of therapy that involves focusing on the relationship between a person’s thoughts, feelings, and actions; often used as part of treatment for sleep disorders such as insomnia. Effective CBT treatment modalities include cognitive therapy, stimulus control therapy, sleep restriction therapy, and relaxation therapy.

Confusional arousals - A type of NREM Sleep Arousal Disorder that occurs when a person wakes up in a confused state.

Continuous Positive Airway Pressure (CPAP) - A treatment modality for sleep-related breathing disorders in which a mask (applied either to the nose or to nose and mouth) is used to open the upper airway by delivering a constant elevated air pressure to the upper airway.

D

Delayed Phase Sleep Disorder (DSPD) - A Circadian Rhythm Disorder characterized by delayed sleep-onset and wake times, compared with societal or preferred norms.

Delta waves - Occur during stage 3 sleep and are the slowest waves with the highest amplitude. Delta sleep is the deepest sleep.

E

Endocrine System - The collection of glands that secrete hormones into the circulatory system and are carried to another organ), which has a complex response to sleep. Secretion of some hormones increases during sleep (e.g., growth hormone, prolactin, and luteinizing hormone), while secretion of other hormones is inhibited during sleep (e.g., thyroid stimulating hormone and cortisol).

Excessive Daytime Sleepiness (EDS) - Sleepiness that interferes with daytime functioning; this can result from sleep deprivation, insomnia, narcolepsy (disabling sleepiness), or cataplexy (sudden muscular weakness). EDS can manifest itself as a tendency to fall asleep during normal waking hours when unstimulated and is objectively diagnosed when the average sleep latency on the Multiple Sleep Latency Test drops below 8 minutes. EDS and “hypersomnia” are currently used interchangeably.

Expiratory Positive Airway Pressure (EPAP) - Treatment device used for obstructive sleep apnea that involves securing a disposable valve inside the person’s nostrils that facilitates airflow.

F

Fatigue - The state of lacking energy and suffering from feelings of exhaustion or listlessness.

G

GABA - The most widespread inhibitory neurotransmitters in the central nervous system, which regulate sleep and other functions including balance, motor coordination, and memory.

Galanin - A neurotransmitter that is distributed throughout the brain; it is also involved in functions beyond sleep, including learning and memory.

H

Hypercapnea - An abnormally high arterial CO2 tension (PaCO2), usually defined as more than 48 mmHg.

Hypersomnia - A synonym for excessive daytime sleepiness. Although persons with hypersomnia chronically fall asleep when unstimulated, they do not necessarily require longer than average hours of sleep.

Hypnagogic Hallucinations - Vivid and dreamlike experiences, usually visual, which occur at sleep onset while the person is still aware.

Hypnic jerks (sleep starts) - Sudden, brief jerks that occur when a person is falling asleep, often associated with a subjective feeling of falling, a sensory flash, or a sleep-onset dream. Sleep starts are benign, but other, more complex motor behaviors may be associated with, or misinterpreted as, sleep starts that are symptomatic of a more serious condition (i.e., Restless Legs Syndrome).

Hypoglossus Nerve Stimulation (HGNS) - Treatment for obstructive sleep apnea that involves the use of an implanted nerve stimulator.

Hypothalamus - An area of the brain with a significant role in promoting wakefulness and sleep; it appears to be a key regulator (or “sleep switch”) of sleep and wakefulness. Wake-promoting hypocretin/orexin neurons are situated in the lateral hypothalamus.

Hypoxemia - Having lower than normal oxygen levels in the arterial blood.

I

Idiopathic hypersomnia - A hypersomnia defined by the presence or absence of long sleep time (i.e., more than 10 hours). Idiopathic hypersomnia is diagnosed when patients have constantly severe excessive daytime sleepiness (EDS) for at least three months without other defining features, such as REM sleep abnormalities or cataplexy.

Insomnia - The most common sleep complaint, involves trouble getting to sleep and/or staying asleep, and/or experiencing sleep that is not refreshing.

Irregular Sleep-Wake Disorder - A Circadian Rhythm Disorder characterized by a lack of well-defined sleep-wake cycle; individuals with IST typically have three or more sleep episodes during a 24-hour period, rather than one major sleep period.

J

Jet Lag Disorder - A Circadian Rhythm Disorder that occurs when there is a temporary mismatch between sleep-wake cycle timing generated by the person’s internal circadian clock and the sleep-wake pattern required by a time-zone change.

K

K-complexes - A sudden increase in wave amplitude that occurs as a person moves from stage 1 to stage 2 sleep.

Kleine-Levin Syndrome (KLS) - Also called “Sleeping Beauty disease,” a hypersomnia characterized by recurrent episodes of hypersomnia in which patients sleep for 12-24 hours a day. Patients may sleep for days, weeks, or months, waking only to eat and/or use the bathroom.

L

Lifestyle - Life stressors often affect a person’s sleep health. Lifestyle issues that negatively impact sleep include: work and/or family stress, being overweight, smoking, using drugs or alcohol, lack of exercise.

Long Sleepers - An individual who sleep longer than average, but feels well and functions without impairment. Long sleep is different from idiopathic hypersomnia, where a person sleeps long hours but still does not feel refreshed.

M

Maintenance of Wakefulness Test (MWT) - A sleep assessment tool that measures the individual’s ability to stay awake under conditions that usually cause sleepiness. Patients typically sit in the dark for 20 or 40 minutes in 2-hour increments throughout the day. To be deemed to be “adequately alert,” an individual must be able to stay awake for an average of at least 11 minutes during the MWT.

Melatonin - A hormone secreted by the pineal gland at levels that peak at night between 2:00 and 4:00 AM, and drop before dawn. The circadian-phase altering effects of melatonin make it useful in treating circadian rhythm sleep disorders.

Mixed Apnea - A condition marked by the signs and symptoms of both obstructive sleep apnea and central sleep apnea.

Multiple Sleep Latency Test (MSLT) - A test that records an individual’s brain electrical activity, eye movements, and changes in chin muscle tonus to measure how rapidly he or she falls asleep and the types of sleep experienced.

N

Narcolepsy - A hypersomnia characterized by profound sleep-onset paralysis and vivid hallucinations that occur as a person is falling asleep (hypnagogic hallucinations). Narcolepsy with cataplexy also involves sudden loss of muscle control (cataplexy).

Night Eating Syndrome (NES) - A condition in which an individual feels hungry, even if he or she has just eaten, and is unable to go to sleep without eating again. NES is not a parasomnia because it occurs when the individual is completely awake. It is different from Sleep-Related Eating Disorder (SRED), which is a parasomnia.

Nightmare - Disturbing mental experiences that generally occur during Rapid Eye Movement (REM) sleep in the later portion of the night, and often result in the person waking up. Occasional nightmares are normal; a nightmare disorder is diagnosed only if nightmares become more frequent and significantly affect daytime functioning and mood.

Non-24-Hour Sleep Wake Disorder - A Circadian Rhythm Disorder characterized by an abnormal sleep-wake rhythm caused by a circadian clock that is not in phase with, and cannot be entrained to, the 24-hour light-dark cycle. It is most frequently experienced by individuals who are blind.

Non-Rapid Eye Movement (NREM) Sleep - One of the three basic states of consciousness, characterized by a reduction in physiological activity. As sleep deepens, a person’s brain waves slow down and gain amplitude, both breathing and the heart rate slow down, and the individual’s blood pressure drops.

Non-Rapid Eye Movement (NREM) Sleep Arousal Disorders - A parasomnia that occurs when the brain is partly in Non-REM sleep and partly awake enough to perform complex activities without any conscious awareness of them. NREM disorders include confusional arousals, sleep-walking, sleep terrors, sleep sex (sexsomnia), and Sleep Related Eating Disorder (SRED).

Noninvasive Positive Pressure Ventilation (NIPPV) - Ventilation applied through a mask rather than through an endotracheal tube. This type of ventilation has a backup mode that delivers automatic pressure changes for inhalation and exhalation when patients do not trigger such pressure changes themselves.

O

Obstructive Sleep Apnea Syndrome (OSAS) - The most common form of sleep apnea. OSAS occurs when the person’s airway collapses or is blocked, which causes shallow breathing (or pauses in breathing) and disrupts sleep.

Occlusal appliances - Devices, such as soft mouth guards or stabilization splints, that are designed to protect the mouth and prevent Sleep-Related Bruxism’s negative outcomes.

Opioids - A class of drugs used for pain management, such as codeine, oxycodone, and morphine.

Oral Appliances - Devices used to treat obstructive sleep apnea that move the tongue or lower jaw forward to increase the size of the upper airway, and facilitate breathing.

Oxygen Desaturation Index (ODI) - Used to assess severity of sleep apnea, the index measures the number of times per hour the person’s oxygen saturation falls by 3 or 4 percent, or the time spent with saturations below 90 percent.

P

Parasomnias - Abnormal activities that can occur during sleep, other than sleep apnea. These activities include sleep-related abnormal movements, behaviors, emotions, and perceptions as well as dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep. Parasomnias include eating disorders, sleepwalking, nightmares, sleep paralysis, REM sleep behavior disorder, and sleep aggression.

Periodic Breathing - A crescendo-decrescendo pattern of respiration, as seen in Cheyne-Stokes and high-altitude breathing, and in premature infants.

Periodic Limb Movement Disorder (PLMD) - A sleep disorder involving repetitive movements of the limb (i.e., arms, legs) that occurs during sleep, and resulting in excessive daytime sleepiness and fatigue.

Periodic limb movements during sleep (PLMS) - Body movements that occur during sleep, usually involving the person’s legs. PLMS are normal and do not cause negative effects.

Periodic Limb Movements Index - A tool used to assess the number of Periodic Limb Movements per hour of sleep, in order to assess a patient for Periodic Limb Movement Disorder (PLMD).

Polysomnography (PSG) - A diagnostic test that monitors a number of physiologic variables during both sleep and wakefulness. The test requires the patient to sleep in the lab overnight, where his/her sleep behavior is monitored by a technician and recorded on video.

Positive Airway Pressure (PAP) - First-line therapy for patients with moderate or severe obstructive sleep apnea. OSAS, the device delivers pressurized air to the upper airway (via a nasal mask, full-face mask, or nasal pillow), which prevents the upper airway from collapsing during sleep. Devices include Continuous Positive Airway Pressure (CPAP) devices, Bilevel Positive Airway Pressure (BPAP) devices, and Autotitrating Positive Airway Pressure (APAP) devices.

Propriospinal myochonus - A parasomnia characterized by brief, involuntary jerks that occur during the transition to sleep. It can be confused with Restless Legs Syndrome or sleep starts.

R

Rapid Eye Movement (REM) Sleep - One of the three basic states of consciousness. An active period of sleep, marked by intense brain activity, rapid breathing, eye movement, and limb muscles paralysis. Most dreams occur during REM sleep.

Rapid Eye Movement Sleep Behavior Disorder (RBD) - A parasomnia in which the typical paralysis that characterizes REM sleep is weakened. People with RBD can act out their dreams, and risk injuring themselves and/or their bed partners.

Rebound (as in restless legs syndrome) - Occurrence or increase in prevalence or severity of symptoms in the morning resulting from withdrawal of a short-acting dopaminergic agent.

Recurrent Isolated Sleep Paralysis - A parasomnia characterized by an inability to talk or move either at sleep onset or upon awakening, while the person is fully conscious. IPS is also characterized by vivid hypnagogic hallucinations.

Relaxation Therapy - Used to treat insomnia by reducing movements and minimizing distracting thoughts, such as focusing on breathing, engaging in muscle relaxation, and meditating.

Respiratory Disturbance Index (RDI) - The number of apneas, hypopneas and Respiratory Effort-related Arousals (RERAs) divided by the hours of sleep.

Respiratory Effort-related Arousal (RERA) - Transient episodes of decreased inspiratory airflow or increased inspiratory effort lasting more than 10 seconds) leading to an arousal. The decreased respiration does not meet the criteria for either an apnea or hypopnea.

Respiratory Effort-related Arousal (RERA) - Transient episodes of decreased inspiratory airflow or increased inspiratory effort lasting more than 10 seconds) leading to an arousal. The decreased respiration does not meet the criteria for either an apnea or hypopnea.

Restless Legs Syndrome (RLS) - A neurological disorder that causes uncomfortable (and sometimes painful) tingling and tugging sensations in the legs. Symptoms usually intensify in the evening and at night, and can interfere with sleep.

Rhythmic Movement Disorder - A parasomnia characterized by repetitive, involuntary movements that occur during sleep, which can result in severe injury, Excessive Daytime Sleepiness, and other negative health outcomes.

Risk factor - Increases a person’s likelihood of developing a disease, experiencing a certain condition, or being injured. Common risk factors include age, gender, socio-economic status, genes, and lifestyle factors (i.e., diet, smoking, alcohol use).

S

Selective Serotonin Reuptake Inhibitors (SSRIs) - A class of drugs typically used to treat depression and anxiety.

Sexsomnia (sleep sex) - A type of NREM Sleep Arousal Disorder that occurs when a person who is asleep engages in sexual acts (this differs from having an erotic dream).

Shift Work Disorder (SWD) - A Circadian Rhythm Disorder characterized by insomnia or excessive sleepiness resulting from a work schedule that interferes with the normal sleep period.

Short Sleepers - An individual who sleeps less than the normal amount, without impairment in daytime functioning; adult short sleepers habitually get 5-6 or fewer hours of sleep per 24 hours.

Sleep Cycles - Sleep is divided into 90-minute cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, repeated three to six times during the night.

Sleep Diary - A log the patient keeps of how many hours slept each night, how many awakenings during the night and their durations, how long taken to fall asleep, how well rested the patient felt upon awakening, and how sleepy the patient felt during the day. Factors affecting sleep, such as the number of caffeinated or alcoholic drinks imbued, and nap or exercise times and lengths, may also be recorded.

Sleep-Disordered Breathing - Conditions during which respiration pauses occur during sleep due to the airways either complete or partial collapse of the person’s airways. These conditions include obstructive sleep apnea (OSA, in which no ventilation occurs) and hypopneas (reduced ventilation due to partial airway obstruction) lead to intermittent and abrupt reduction in blood oxygen levels. Symptoms include snorting, snoring, gasping, and choking in one’s sleep.

Sleep Enuresis (Bedwetting) - A parasomnia defined as recurrent, involuntary voiding of urine during sleep. Sleep enuresis is diagnosed as a disorder if it occurs chronically (i.e., at least twice per week) and does not respond to conventional therapies.

Sleep Habits - Practices that foster and maintain good sleep health. Good sleep habits include behaviors and conditions that can be consciously changed to improve an individual’s quality and quantity of sleep.

Sleep Latency - How fast a person falls asleep.

Sleep Paralysis - The paralysis experienced during sleeping that persists after awakening or just prior to falling asleep. Paralysis at sleep onset is often associated with narcolepsy; paralysis at wake onset is not.

Sleep-Related Breathing Disorders - Disorders caused by a reduction or suspension in breathing during sleep; these disorders are a primary cause of excessive daytime sleepiness. The most common types are Obstructive Sleep Apnea Syndrome (OSAS), Central Sleep Apnea Syndrome, and Sleep-Related Hypoventilation Syndromes (SRHS).

Sleep-Related Bruxism - A sleep disorder characterized by grinding and/or clenching the teeth during sleep, usually also associated with sleep arousals.

Sleep-Related Eating Disorder (SRED) - A type of NREM Sleep Arousal Disorder that involves recurrent episodes of involuntary eating and/or drinking during sleep, usually during partial arousals. Consumption of peculiar forms or combinations of food or substances is typical (e.g., cigarette and peanut butter sandwich).

Sleep-Related Groaning (catathrenia) - A parasomnia characterized by groaning or moaning sounds, which are usually clustered and occur within a long period of expiration (2-50 seconds) subsequent to a deep inspiration.

Sleep-Related Hallucinations - Realistic hallucinations that occur during sleep onset (hypnagogic) or upon awakening from sleep (hypnopompic), and that can be very frightening. They usually consist of complex visual images, although auditory, tactile, or kinetic events may also occur.

Sleep-Related Hypoventilation Syndromes (SRHS) - A type of sleep disorder characterized by abnormal ventilation and gas exchange that significantly worsens during sleep, or may only be present during sleep. The abnormalities result in increased blood carbon dioxide (CO2) levels (hypercapnea) and are often associated with low blood oxygen concentrations (hypoxemia).

Sleep-Related Leg Cramps - Painful sensations caused by sudden and intense muscle contractions, usually in the calf or the small muscles of the foot, which occur during sleep or wakefulness.

Sleep-Related Movement Disorders - Disorders diagnosed when an individual experiences insomnia, excessive daytime fatigue, or non-restorative sleep stemming from movements that occur either during sleep or near sleep onset.

Sleep-Related Rhythmic Movement Disorder (RMD) - A sleep-related movement disorder that involves large and stereotyped rhythmic movements, including head banging, head rolling, rocking on the elbows and knees, rocking and/or rolling the body from side to side.

Sleep Sex (sexsomnia) - A type of NREM Sleep Arousal Disorder that occurs when a person who is asleep engages in sexual acts (this differs from having an erotic dream).

Sleep Spindles - A sudden increase in wave frequency that occurs as a person moves from stage 1 to stage 2 sleep.

Sleep Starts (hypnic jerks) - Sudden, brief jerks that occur when a person is falling asleep, often associated with a subjective feeling of falling, a sensory flash, or a sleep-onset dream. Sleep starts are benign, but other, more complex motor behaviors may be associated with, or misinterpreted as, sleep starts that are symptomatic of a more serious condition (i.e., Restless Legs Syndrome).

Sleep Talking (somniloquy) - Talking that occurs during sleep, ranging from mumbling to uttering clear words and/or sentences.

Sleep Terrors - A type of NREM Sleep Arousal Disorder include occurs when a sleeping person appears to be awake, and experiences profound inconsolability and panic (often including hitting or running around).

Sleep-Walking - A type of NREM Sleep Arousal Disorder that occurs when a person sits up and/or gets out of bed and walks around while they are asleep.

Slow-Wave Sleep (SWS) - A stage of NREM sleep characterized by the presence of slow brain waves called “delta waves” interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and temperatures drops even lower, with the body becoming immobile. Sleep is deeper, with no eye movement and decreased muscle activity, although muscles retain their ability to function.

Somniloquy (sleep talking) - Talking that occurs during sleep, ranging from mumbling to uttering clear words and/or sentences.

Stage 1 Sleep - A stage of Non-Rapid Eye Movement (NREM) sleep; a time of drowsiness or transition from being awake to falling asleep. Brain waves and muscle activity begin slowing down in this stage. People in stage 1 sleep may experience sudden muscle jerks, preceded by a falling sensation.

Stage 2 Sleep - A stage of Non-Rapid Eye Movement (NREM) sleep; a period of light sleep during which eye movements stop. Brain waves become slower, with occasional bursts of rapid waves (called sleep spindles) and spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and body temperature decreases.

Stage 3 Sleep - A stage of Non-Rapid Eye Movement (NREM) sleep, also called “slow wave sleep” (SWS); characterized by the presence of slow brain waves called “delta waves” interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and temperatures drops even lower, with the body becoming immobile.

Stimulus Control Therapy - A type of insomnia treatment that focuses on engaging in actions that reinforce a person’s association of bedtime and the bedroom with rapid sleep onset.

Suprachiasmatic Nucleus (SCN) - A group of cells located in the hypothalamus immediately above the optic chiasm, which receives input from the retina. The SCN generates a circadian wakefulness-promoting signal keeps people awake during the day and provides circadian rhythmicity to nearly every physiological system.

T

Theta waves - Occur during stages 1 and 2 and are slower in frequency and greater in amplitude than alpha waves. As a person moves from stage 1 to stage 2 sleep, theta wave activity continues; every few minutes, sleep spindles (sudden increase in wave frequency) and K-complexes (sudden increase in wave amplitude) occur.

Tolerance - The need to provide increasing doses of medication to provide the same relief.

Total sleep time (TST) - The actual amount of time spent sleeping from sleep onset until awakening; TST includes all REM and NREM sleep.

U

Upper Airway Resistance Syndrome (UARS) - A subclass of Obstructive Sleep Apnea Syndrome, diagnosed when the complaint of unrefreshing sleep and daytime somnolence is associated with frequent RERAs, but not with apneas or hypopneas.

V

Ventrolateral Preoptic Area (VLPO) - An area of the hypocampus where sleep-promoting GABA/galanininergic (GABA/Gal) neurons are located.

W

Wake After Sleep Onset (WASO) - The number of times a person wakes up during sleep, which increases with age.

Wrist Actigraph - A small device that monitors and analyses wrist movements during routine daily activities to estimate time spent sleep vs. awake. Wrist actigraphy provides a fair (although not exact) measurement of when, and for how long, a patient sleeps.