rhythm, biological, cyclic pattern of physiological changes or changes in activity in living organisms, most often synchronized with daily, monthly, or annual cyclical changes in the environment. The exact nature of the internal mechanism, or "biological clock," that controls such rhythms is not understood. Rhythms that vary according to the time of day, called circadian rhythms, include such phenomena as the opening and closing of flowers and, in humans, changes in body temperature, blood pressure, and urine production. In diurnal animals, activity increases in daylight; in nocturnal animals nighttime activity predominates. Activity of many marine organisms varies according to the tide. Monthly rhythms include weight changes in men and the menstrual period in women. Annual cycles, or circannual rhythms, include bird migrations, reproductive activity, and mammalian hibernation. Daily cycles, or circadian rhythms, are in part a response to daylight or dark, and annual cycles in part responses to changes in the relative length of periods of daylight. However, environmentally determined cyclical changes, such as changes in daylight, temperature, and availability of food, serve primarily to refine and adjust physiologically determined circadian or circannual rhythms: in the absence of external cues, the internal rhythms gradually drift out of phase with the environment. Physiological rhythms are also present in the activity of individual organs, e.g., the beating of heart muscle and the activity of electrical waves of the brain.

rhythm, the basic temporal element of music, concerned with duration and with stresses or accents whether irregular or organized into regular patternings. The formulation in the late 12th cent. of the rhythmic modes—basic recurrent patterns that were adhered to in composition—began the development of the Western system of meter and its notation. Most rhythms are metrical, i.e., the values are multiples of a temporal unit, or beat, usually associated with some particular note value. Free rhythm, such as occurs in much Asian music, has no meter (i.e., its temporal values are not derived from a basic unit). The degree of rhythmic complexity and the types of rhythms used are major considerations in analysis of the style of a composer or a period. The rhythmic tension of music is of value in eliciting emotional response from the hearer. African music and some 20th-century composers employ polyrhythm, the simultaneous use of several rhythmic patterns whose accents do not coincide. See syncopation and metronome.

circadian rhythm: see rhythm, biological.

Poetic rhythm designed to approximate the natural rhythm of speech. It is characterized by the frequent juxtaposition of single accented syllables and by the occurrence of feet with varying numbers of syllables whose sequence is interrupted by unstressed syllables that are not counted in the scansion. Because stressed syllables often occur sequentially, the rhythm is said to be “sprung.” This system of prosody was developed by Gerard Manley Hopkins, who saw it as the basis of such early English poems as William Langland's Piers Plowman.

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Periodic biological fluctuation in an organism corresponding to and in response to periodic environmental change, such as day and night or high and low tide. The internal mechanism that maintains this rhythm even without the apparent environmental stimulus is a “biological clock.” When the rhythm is interrupted, the clock's adjustment is delayed, accounting for such phenomena as jet lag when traveling across time zones. Rhythms may have 24-hour (circadian rhythm), monthly, or annual cycles. Seealso photoperiodism.

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Any of several closely related musical styles developed by African American artists. The various styles were based on a mingling of European influences with jazz rhythms and tonal inflections, particularly syncopation and the flatted blues chords. They grew out of the blues of the rural South, which blended work chants with songs of deep emotion, and were greatly influenced by gospel music. Three major forms were distinguishable. The earliest, called race, was the style of the “jump” band, which emphasized strong rhythm, solo work (especially by saxophones), and vocals in a shout-blues manner. A second form, often called Chicago blues, was exemplified by performers such as Muddy Waters and was typically played by a small group with amplified instruments. The third major form was primarily vocal, featuring close, gospel-influenced harmonies often backed by an orchestra. In the mid-1950s the term rhythm and blues was adopted by the music industry for music intended for the African American audience; with the gradual disappearance of racial barriers, the Chicago blues style began to seem less a vital form than a folk tradition, while the gospel style was transformed into the soul music of vast appeal. Rhythm and blues was the chief antecedent of rock music.

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Inherent cycle of approximately 24 hours in length that appears to control or initiate various biological processes, including sleep, wakefulness, and digestive and hormonal activity. The natural signal for the circadian pattern is the change from darkness to light. The controlling mechanism for these cyclic processes within the body is thought to be the hypothalamus. Any change in the circadian cycle (such as jet lag and other conditions associated with travel) requires a certain period for readjustment.

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