Sleep Disorders Glossary F

Sleep Disorders Glossary - F

"Factor S"

"Factor S" is a substance in the cerebrospinal fluid that has sleep-inducing properties.


Fatigue is physical or mental weariness resulting from exertion. Fatigue is the feeling of tiredness, weariness or lack or energy usually associated with lower performance. Fatigue is different from drowsiness. Fatigue often develops in response to physical exertion, emotional stress, boredom, or lack of sleep.

Fiber-Optic Nasopharyngoscope

Fiber-Optic Nasopharyngoscope is a flexible fiber optic scope used to examine the nasal passages, hypopharynx, pharynx, and larynx.


Fibromyalgia is a medical syndrome that causes widespread pain and stiffness in the muscles and joints as well as sleeps problems and chronic daytime fatigue. The most common sites of pain include the neck, back, shoulders, pelvic girdle and hands, but any body part can be involved.

Flattening Index

Flattening Index is used to identify a condition known as Upper Airway Resistance Syndrome (UARS), and is continuously recorded in some diagnostic sleep studies and CPAP titration studies.

0.3 indicates an open airway, 0.15 is mildly obstructed, 0.1 is severely limited airflow, and 0.0 reflects a totally closed airway.

Flow Limitation

Flow Limitation is the partial closure of the upper airway, which obstructs the flow of air into the lungs.

Forbidden Zone

Forbidden zone is the period of strongest clock-dependent alerting, usually in the evening, which prevents falling asleep.


Fragmentation is pertaining to sleep architecture. It is the interruption of any stage of sleep due to appearance of another stage or waking, transition to a lighter stage, or to wakefulness, leading to disrupted sleep cycles.

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