circadian rhythm

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a measured movement; the recurrence of an action or function at regular intervals. adj., adj rhyth´mic, rhyth´�mical.
accelerated idiojunctional rhythm a junctional rhythm, without retrograde conduction to the atria, at a rate exceeding the normal firing rate of the junction; it is an ectopic rhythm located in the bundle of His and controlling ventricles at a rate of 60 to 100 beats per minute.
accelerated idioventricular rhythm a rhythm of ectopic ventricular origin, faster than the normal rate of the His-Purkinje system but slower than 100 beats per minute, without retrograde conduction to the atria.
accelerated junctional rhythm a rhythm emanating from a focus in the AV junction at a rate greater than its normal rate of 60 but less than 100 beats per minute; it may be due to altered automaticity secondary to disease or to triggered activity secondary to digitalis toxicity. There may or may not be retrograde conduction to the atria.
alpha rhythm uniform rhythm of waves in the normal electroencephalogram, showing an average frequency of 10 per second, typical of a normal person awake in a quiet resting state. Called also Berger rhythm. See also electroencephalography.
atrioventricular junctional rhythm a junctional rhythm originating in the bundle of His, with a heart rate of 40 to 60 beats per minute; called also nodal rhythm.
automatic rhythm spontaneous rhythms initiated by the sinoatrial node, or by subsidiary atrial or ventricular pacemakers; in practice this refers to a normal sinus rhythm at a rate of 60 to 100 beats per minute.
Berger rhythm alpha rhythm.
beta rhythm a rhythm in the electroencephalogram consisting of waves smaller than those of the alpha rhythm, having an average frequency of 25 per second, typical during periods of intense activity of the nervous system. See also electroencephalography.
biological r's the cyclic changes that occur in physiological processes of living organisms; these rhythms are so persistent in nature that they probably should be considered a fundamental characteristic of life, as are growth, reproduction, metabolism, and irritability. Many of the physiological processes that recur in humans about every 24 hours (with circadian rhythm) have been known for centuries. Examples include the peaks and troughs seen in body temperature, vital signs, brain function, and muscular activity. Biochemical analyses of urine, blood enzymes, and plasma serum also have demonstrated circadian rhythms. Called also biorhythms.

It has long been believed that the cyclic changes observed in plants and animals were totally in response to environmental changes and, as such, were exogenous or of external origin. This hypothesis has now been rejected by most chronobiologists, who hold that the biological rhythms are intrinsic to the organisms, and that the organisms possess their own physiological mechanism for keeping time. This mechanism has been called the “biological clock.” An example of adjustment of the biological clock in humans is recovery from “jet lag.” This phenomenon, also known as jet syndrome, occurs when humans are transported by jet plane across time zones. It is characterized by fatigue and lowered efficiency, which persist until the biological clock adjusts to the new environmental cycle.

Biological rhythms are responsive to, or synchronous with, environmental cycles, but it is generally agreed among chronobiologists that the rhythmic changes in environmental factors do not create biological rhythms, even though they are capable of influencing them. Even in the absence of such environmental stimuli as light, darkness, temperature, gravity, and electromagnetic field, biological rhythms continue to maintain their cyclic nature for a period of time.
circadian rhythm the regular recurrence in cycles of about 24 hours from one point to another, such as certain biological activities that do this regardless of long periods of darkness or other changes in environmental conditions.
circamensual rhythm recurrence in cycles of about one month (30 days).
circannual rhythm recurrence of a phenomenon in cycles of about one year.
circaseptan rhythm that which occurs in cycles of about seven days (one week).
coupled rhythm heart beats occurring in pairs, the second beat of the pair usually being a ventricular premature beat.
delta rhythm
1. electroencephalographic waves having a frequency below 3½ per second, typical in deep sleep, in infancy, and in serious brain disorders. See also electroencephalography.
2. delta waves.
escape rhythm a heart rhythm initiated by lower centers when the sinoatrial node fails to initiate impulses, its rhythmicity is depressed, or its impulses are completely blocked.
gallop rhythm an auscultatory finding of three or four heart sounds, created by gushes of blood entering resistant or stiffened ventricles. This can happen at two different times during ventricular diastole: either at initial filling or at the time of ventricular contraction. Therefore, gallops occur during early and late ventricular diastole.
gamma rhythm a rhythm in the waves in the electroencephalogram having a frequency of 50 per second. See also electroencephalography.
idiojunctional rhythm a rhythm emanating from the atrioventricular junction but without retrograde conduction to the atria.
infradian rhythm the regular recurrence in cycles of more than 24 hours, as certain biological activities which occur at such intervals, regardless of conditions of illumination or other environmental conditions.
junctional rhythm an arrhythmia caused by an abnormality in the atrioventricular junction; see accelerated junctional rhythm and atrioventricular junctional rhythm.
rhythm method old popular name for natural family planning.
nyctohemeral rhythm a day and night rhythm.
pendulum rhythm alternation in the rhythm of the heart sounds in which the diastolic sound is equal in time, character, and loudness to the systolic sound, the beat of the heart resembling the tick of a watch.
sinus rhythm normal heart rhythm originating in the sinoatrial node, with a normal rate of 60 to 100 beats per minute.
theta rhythm electroencephalographic waves having a frequency of 4 to 7 per second, occurring mainly in children but also in adults under emotional stress. See also electroencephalography.
ultradian rhythm the regular recurrence in cycles of less than 24 hours, as certain biological activities which occur at such intervals, regardless of conditions of illumination or other environmental conditions.
ventricular rhythm the ventricular contractions which occur in cases of complete heart block.


Relating to biologic variations or rhythms with a cycle of about 24 hours. Compare: infradian, ultradian.
[L. circa, about, + dies, day]

circadian rhythm

A daily rhythmic activity cycle, based on 24-hour intervals, that is exhibited by many organisms.

circadian rhythm

The diurnal cadence which, in humans (without cyclical cues provided by natural light), is 24.1–24.2 hours (older data had put this figure at 25.4 hours). Photoperiodic information from the eyes synchronises the circadian pacemaker with the light-dark cycle; circadian rhythm affects drug metabolism (e.g., antacids, halothane), serum levels of various substances (in particular adrenocortical hormones) that are routinely measured to detect and monitor disease, physiologic activities (e.g., blood pressure, myocardial blood flow and oxygen demand), psychosomatic disease and sleep cycles, cell division, haematopoiesis and natural killer cell activity.

circadian rhythm

Diurnal rhythm, ultradian rhythm Physiology An innate, daily, fluctuation of physiologic and behavioral functions–eg sleep waking, generally tied to the 24 hr day-night cycle; the diurnal cadence in humans without cyclical cues provided by natural light, is 25.4 hrs; CR affects drug metabolism, serum levels of various substances–eg, ACTH, physiologic activities–eg, BP, myocardial blood flow and O2 demand, psychosomatic disease and sleep cycles, cell division, hematopoiesis, NK cell activity. See Biorhythm, Circadian pacemaker, Insomnia, Jet lag, Melatonin, Shift work.

circadian rhythm

A biological 24 hour cycle that applies to many physiological processes and variables and is synchronized to the day-night cycle occasioned by the rotation of the earth. See also BIORHYTHMS.

circadian rhythm

the basic rhythm with a periodicity of approximately 24 hours that organisms undergo when isolated from the daily rhythmical changes of the environment and are, for example, kept entirely in the dark. This rhythm demonstrates the ability of the organs to measure time, but the physiological basis of this is largely unknown. See DIURNAL RHYTHM, BIOLOGICAL CLOCK.

Circadian rhythm

Any body rhythm that recurs in 24-hour cycles. The sleep-wake cycle is an example of a circadian rhythm.

rhythm, circadian 

The characteristic of some processes to repeat at approximately 24-hour intervals. Examples: intraocular pressure is at its lowest every evening and highest every morning; corneal sensitivity is at its lowest every morning and highest every evening. Syn. diurnal cycle (provided the variation in activity or behaviour is more or less divided equally between night and day). See melanopsin.
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