Actigraph: A wrist-watch like device to be used to monitor body movements in sleep. Helps to estimate sleep and wake time. It is a clinical and research tool.

Advanced Sleep Phase syndrome (ASPS): Common in elderly with a tendency to go to bed early (e.g. 6 PM) and wake up early (e.g. 3 AM). This is a dysfunction of circadian rhythm.

Alpha sleep: Alpha activity noted during most, if not all, stages of sleep.

Apnea: A period of cessation of breathing for at least 10 or more seconds. Divided into obstructive, mixed and central types.

Apnea index: The number of apneic episodes per hour of sleep, calculated from all night Polysomnography.

Arousal: Abrupt change from deeper stage of sleep to lighter stage or from REM sleep toward wakefulness .

Bedtime: The clock time when one attempts to fall asleep as differentiated from the clock time when one gets into bed.

Bright light therapy: Used to treat circadian rhythm disorders. Blue light is said to be more effective than others.

Brauxism: Grinding of teeth in sleep

Cataplexy: A sudden loss of muscle tone brought on by emotions, anger, excitement or laughter, a characteristic feature of narcolepsy.

Central sleep apnea: A cessation of breathing without obstruction of airway with loss of movements of chest and abdomen. Often seen in heart failure.

Chronotherapy: A method of treatment of circadian rhythm disorders. The bed time is gradually adjusted over several days until an acceptable sleep-wake cycle is achieved.

Cheyne-Stokes breathing: Abnormal breathing pattern with a gradual increase and a gradual decrease in depth and rate of breathing.

Circadian rhythm: Internal clock (suprachiasmatic nucleus) that influences sleep-wake cycle and other physiological functions.

Circasemidian rhythm: A biological rhythm that has a period length of about half a day.

Cognitive behavioral therapy (CBT): A modality of treatment of insomnia.

CPAP: Continuous positive airway pressure, an effective method of treatment of obstructive sleep apnea.

Chronobiology: The science relating to temporal, primarily rhythmic, processes in biology.

Conditioned insomnia: An insomnia that is produced by the development of conditioned arousal during an earlier experience of sleeplessness. A component of psychophysiological insomnia.

Delta sleep: Also known as deep sleep, classified as N3 stage of NREM sleep.

Delayed Sleep Phase Syndrome (DSPA): A circadian rhythm sleep disorder where in the sleep phase occurs well beyond the conventional bed time with delayed wake up time. Seen in teenage groups and young adults (Night owl).

Diurnal: Pertaining to daytime.

Epworth Sleepiness Scale (Epworth Sleepiness Score): A subjective assessment of severity of daytime sleepiness. A very commonly used measure of excessive daytime sleepiness.

Final awakening: The amount of wakefulness that occurs after the final wake-up time until the arise time (lights on).

Final wake-up: The clock time at which an individual awakens for the last time before the arise time.

First night effect: The effect of the environment and Polysomnographic recording apparatus on the quality of the subject’s sleep the first night or recording.

Free running: A Chronobiological term that refers to the natural endogenous period of a rhythm when zeitgebers are removed.

Hypercapnia: Elevated carbon dioxide level in blood.

Hypersomnia: Excessive sleepiness

Hypnogram: A graphic representation of various stages of NREM and REM sleep.

Hypnogogic hallucinations: Hallucinations occurring at the transition from wakefulness to sleep and or at sleep onset.

Hypnopompic hallucinations: Hallucinations occurring at the transition from sleep to wakefulness or at the offset of sleep.

Insomnia: Difficulty to initiate sleep (DIS) or maintain sleep (DMS) despite adequate opportunity to sleep.

Jet lag: Symptoms due to asynchrony between habitual sleep time and local sleep time of destination.

Light-Dark cycle: The periodic pattern of light alternating with darkness.

Maintenance of wakefulness test (MWT): A test to measure an ability to keep awake in a non-stimulating environment (4 to 5 trials).

Mean Sleep Latency Test (MSLT): A test to measure an ability to fall asleep in a sleep promoting environment.

Melatonin: A neuron-hormone released from pineal gland and is involved in the regulation of circadian rhythm.

Micro sleep: An episode lasting for 30 seconds during which external stimuli are not perceived. Micro sleeps are associated with excessive sleepiness and automatic behavior and accidents.

Nap: A short sleep episode that may be intentional or unintentional occurring during habitual wakefulness.

Narcolepsy: A sleep disorder with excessive sleepiness, cataplexy, sleep paralysis and hypnogogic hallucinations.

Night terror: Parasomnia seen in children with awakenings at night with fear and terror. It occurs in NREM sleep.

Nightmare: Parasomnia with terrifying dreams.

Nocturnal enuresis: Seen in children with urination in sleep (Bed wetting).

NPT: The natural periodic cycle of penile erections that occurs during sleep.

NREM: Non-rapid eye movement state of sleep consisting of N1, N2 and N3 stages (75% of night time sleep).

Obstructive Sleep Apnea: Breathing difficulty in sleep due to collapse of airway.

Periodic limb movement disorder: Involuntary jerking of limbs in sleep with fragmentation of sleep associated with daytime dysfunction.

Penile buckling pressure: The amount of force applied to the glans of penis that is sufficient to produce at least a 30 degree bend in the shaft.

Parasomnias: Abnormal behavior in NREM and REM sleep associated with injury, interruption of sleep, insomnia and daytime dysfunction.

Polysomnography: A sleep test that monitors EEG, EKG, leg movements, Eye movements, breathing patterns, snoring and oxygen saturation.

Quiet sleep: A term used for describing NREM sleep in infants when specific NREM sleep stages N1 to N3 cannot be determined.

Rapid eye movement sleep (REM sleep): Also known as dream state of sleep or paradoxical sleep with rapid eye movements with paralysis of rest of the body, irregular breathing and change in heart rate.

Restless leg syndrome: A condition with sensory symptoms in limbs and irresistible desire to move legs especially when resting and worse at night. Leg jumps may occur in sleep (Karate kid).

Sleep architecture: A division of sleep into N1, N2 and N3 and REM sleep that can be quantified.

Sleep apnea: A disorder where a person’s breathing is interrupted partially or completely.

Sleep cycle: A cycle of NREM and REM sleep that lasts 80 to 110 minutes. There may be 4 to 6 such cycles in each night.

Sleep efficiency: A ratio of total sleep time to total bed time.

Sleep hygiene: A methodology to obtain a good quality of sleep.

Sleep latency: Time in minutes required to fall in to sleep after going to bed.

Sleep onset: The transition from wakefulness to sleep.

SOREM: The beginning of sleep by entrance directly into REM stage of sleep.

Sleep paralysis: A reversible state of bodily paralysis noted in narcolepsy and other sleep disorder.

Sleep talking: A Parasomnia with talking in sleep, often appears in NREM sleep.

Snoring: A noise produced primarily with inspiratory cycle of breathing in sleep due to vibration of soft palate and pillars of oropharyngeal inlet.

Somnambulism: Sleep walking common among children, occurs in NREM sleep (especially N2).

Spindle REM sleep: Atypical appearance of sleep spindles in REM sleep, seen in chronic insomnia.

Suprachiasmatic nucleus: A structure in hypothalamus that regulates sleep and wake cycle.

Total recording time: The duration of time from sleep onset to final awakening.

Total sleep episode: Equivalent to total sleep period. A combination of NREM, REM and wakefulness in between.

Total sleep time: The actual amount of sleep in total sleep period (a combination of NREM and REM sleep).

Tumescence: Penile tumescence is hardening and expansion of penile in REM sleep.

Wake time: The total time occurring between sleep onset and final wake up time that is scored as wakefulness in polysomnogram.

Zeitgeber: An environmental time cue such as sunlight, noise, social interaction, alarm clocks that usually helps an individual entrain to the 24-hour day.