in a language, a category of words that readily admits new members, such as those arising through borrowing, word formation, or technological innovation. In practice, this category is virtually identical with the category of content words because it usually excludes grammatical function words. Compare closed-class words.
one of four basic types of sleep disorders, differentiated from the other types in that it results from a mismatch between one’s internal circadian rhythm and one’s actual sleep schedule. The equivalent classification in DSM–IV–TR is circadian rhythm sleep disorder (or in DSM–5, circadian rhythm sleep–wake disorder). Rotating work-shift schedules and jet lag are two common causes of this disorder. Diagnosis can involve observation in a sleep laboratory, in which such criteria as nocturnal awakening, sleep time, sleep efficiency, breathing patterns, body temperature, minutes of REM sleep, and sleep latency are measured.