Antihistamine
A type of chemical present in many cold and allergy medicines, and some nonprescription sleep medicines. When taken at night as sleep aids, antihistamine-containing medicines may result in daytime drowsiness.

Apnea
A temporary pause in one's breathing pattern. Sleep apnea consists of repeated episodes of temporary pauses in breathing during sleep.

Circadian rhythm
From the Latin words "circa" and "dies," or "around day".  Any body pattern that follows a twenty-four-hour cycle, such as waking and sleeping.

Continuous Positive Airway Pressure (CPAP) 
This is a very effective therapy in which a mask is worn during the night. It provides a specific pressure that will allow the person to breathe and sleep normally throughout the night.

Cortisol
A steroid hormone produced by the adrenal cortex that mediates various metabolic processes, has anti-inflammatory and immunosuppressive properties, and whose levels in the blood may become elevated in response to physical or psychological stress.

Dyssomnias
One of three broad classification of sleep disorders used in the International Classification of Sleep Disorders (ICSD).  Dyssomnias are disorders that produce either insomnia or excessive sleepiness.  These are the major disorders associated with disturbed sleep and include primary insomnia, obstructive sleep apnea, narcolepsy, restless legs syndrome, and various circadian rhythm disorders.

Growth hormone
A hormone that is secreted by the anterior lobe of the pituitary gland and regulates growth.

Hypersomnia 
A condition marked by excessive sleepiness during normal waking hours.

Hypopnea 
Shallow breathing.

Insomnia 
Sleep problems characterized by difficulty falling asleep, and/or staying asleep. Insomnia can result in getting up in the morning feeling unrested and experiencing drowsiness during the day.

Jet lag
A temporary disruption of the body's sleep/wake rhythm caused by high-speed air travel through different time zones.

Medical-psychiatric sleep disorders
One of three broad classification of sleep disorders used in the International Classification of Sleep Disorders (ICSD).

Melatonin 
A naturally occurring hormone associated with sleep. Synthetic forms of melatonin are sold as sleep aids, although clinical data supporting melatonin use are insufficient.

Metabolism
The chemical processes occurring within a living cell or organism that are necessary for the maintenance of life. In metabolism, some substances are broken down to yield energy for vital processes while other substances, necessary for life, are synthesized.

Microsleeps 
Fleeting, uncontrollable episodes of sleep occurring when one is awake. Although they may last only a few seconds, microsleeps can cause car accidents, machinery-related accidents, and other dangerous conditions.

Narcolepsy 
A sleep disorder marked by sudden, uncontrollable urges to sleep, causing an individual to fall asleep at inappropriate times.
 
Neuronal connections 
Information from one neuron flows to another neuron across a synapse. The synapse is a small gap separating neurons.

Nightmare 
A frightening dream that can cause an individual to wake up. Nightmares typically occur during REM sleep.

NREM sleep 
Non-rapid eye movement sleep. A stage of sleep in which brain activity and bodily functions slow down. NREM sleep accounts for the largest portion of the sleep cycle. See REM sleep.

Obstructive sleep apnea 
One of the more common sleep disorders. The most common symptoms include loud snoring, sleepiness during the day, high blood pressure and obesity. A bed partner will often report that the patient will stop breathing during the night. Untreated sleep apnea may cause cardiovascular disease, risk of automobile accidents, and decreased quality of life and work.

Oxidative stress 
Molecules called free radicals can damage human cells, including brain cells. These molecules form naturally whenever the body metabolizes oxygen, so the damage they cause is called oxidative stress.

Parasomnias
One of three broad classification of sleep disorders used in the International Classification of Sleep Disorders (ICSD).  Parasomnias consist of the disorders that are undesirable physical phenomena that occur during sleep. Generally they are disorders of arousal, partial arousal, and sleep stage transitions. These disorders include sleepwalking, sleep terrors (night terrors), REM behavior disorder, and rhythmic movement disorders.

Periodic limb movements 
During sleep (PLMS) are kicking movements of the legs which occur every 20-90 seconds during the night and may disturb the normal sleep of both you and your partner.

Polysomnograph
An instrument used to measure a patient's body processes during sleep.

Progressive relaxation 
A technique that involves relaxing the body's muscle groups in a progressive sequence, usually starting at the toes and moving upward toward the head.

REM sleep 
Rapid eye movement sleep. REM sleep occurs in brief spurts of increased activity in the brain and body. REM is considered the dreaming stage of sleep. It is characterized by the darting of the eyes under the eyelids.

Restless leg syndrome
A condition in which a patient experiences aching or other unpleasant sensations in the calves of the legs.

Shift work sleep disorder (SWSD)  
Shift work sleep disorder is a sleep disorder that affects people who frequently rotate shifts or work at night, contrary to the body's natural circadian rhythms, and cannot adjust to their schedule.

Sleep 
A periodic state of rest during which consciousness of the world is interrupted.

Sleep apnea 
A condition characterized by temporary breathing interruptions during sleep. The pauses in breathing can occur dozens or even hundreds of times a night. Symptoms include loud snoring and a gasping or snorting sound when the sleeping individual starts to breathe again. Although the individual may not be aware of having sleep apnea, the condition can disrupt the quality of sleep and result in daytime fatigue. The most common type, obstructive sleep apnea, occurs when the tongue or other soft tissue blocks the airway.

Sleep center 
A laboratory or other clinical facility equipped to diagnose sleep disorders.

Sleep cycle 
A sequence of sleep stages that usually begins with a period of about 80 minutes of NREM sleep followed by about 10 minutes of REM sleep. This cycle of approximately 90 minutes is repeated four to six times each night. If the sequence is interrupted (for example, by external noise or a sleep disorder), the quality of sleep can suffer.

Sleep disorders 
A group of syndromes characterized by disturbance in the patient's amount of sleep, quality or timing of sleep, or in behaviors or physiological conditions associated with sleep.

Snoring 
Hoarse, often loud breathing sound that occurs during sleep. Snoring often occurs when the soft palate in the mouth vibrates. Snoring may be a sign of sleep apnea.
 
Somnambulism
Also called sleepwalking, it refers to a range of activities a patient performs while sleeping, from walking to carrying on a conversation.

Stimulant
A substance that makes a person feel more energetic or awake. A stimulant may increase organ activity in the body.

Valerian 
Valerian is an herb sold as a dietary supplement and is a common ingredient in products promoted as mild sedatives and sleep aids for nervous tension and insomnia.  Evidence from clinical studies of the efficacy of valerian in treating sleep disorders such as insomnia is inconclusive.