chronobiology Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronob ...

, a circasemidian rhythm is a physiological arousal cycle that peaks twice in a 24-hour day. It may also be called the semicircadian rhythm . Numerous studies have demonstrated that human

circadian rhythms A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...

in many measures of performance and physiological activity have a 2-peak daily (circasemidian) pattern. The word, circasemidian, is based upon the

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

words ''circa'' ("about"), ''semi'' ("half") and ''dia'' ("day"). Thus, this is a rhythm that has two cycles per day, and some investigators have referred to it as the semicircadian rhythm. It usually serves to (1) deepen the pre-dawn

nadir The nadir (, ; ar, نظير, naẓīr, counterpart) is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direc ...

body temperature Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...

and cognitive performance, (2) create a flat spot during the early afternoon in the daytime increase in body temperature and cognitive performance (the "post-lunch dip"), and (3) heighten the early-evening peak in body temperature and cognitive performance. Broughton was the first to bring this characteristic of human performance to the attention of researchers.

Evidence

No evidence exists to support the presence of a circasemidian rhythm in the rhythmic cells of the

suprachiasmatic nucleus The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regu ...

, the accepted internal timing source for the major circadian rhythms of the body. Thus, though it falls in the domain of an

ultradian rhythm In chronobiology, an ultradian rhythm is a recurrent period or cycle repeated throughout a 24-hour day. In contrast, circadian rhythms complete one cycle daily, while infradian rhythms such as the human menstrual cycle have periods longer than ...

, the circasemidian rhythm may be the first

harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', t ...

of the circadian rhythm and not an intrinsic rhythm. However, a number of published data sets have shown a daily two-peak error pattern in industrial and transportation environments. The pattern was also obvious in many of the charts shown in the review by Rutenfranz and Colquhoun, though they did not suggest a circasemidian rhythm as a mediator for the pattern. Other investigators have reported a circasemidian rhythm in body temperature, melatonin and slow-wave sleep.Hayashi M, Morikawa T, Hori T. "Circasemidian 12 h cycle of slow wave sleep under constant darkness". ''Clin Neurophysiol'' 113(9):1505-16, 2002.

Recommendation

These behavioral and physiological observations support the need to consider a 12-hour rhythmicity in the quantification of daily variations in physiological function and some kinds of cognitive performance in

fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...

modeling efforts such as the

Fatigue Avoidance Scheduling Tool Fatigue is a major human factors issue in aviation safety.Caldwell JA, Caldwell JL. Fatigue in military aviation: an overview of US military-approved pharmacological countermeasures. ''Aviat Space Environ Med'' 76(7, Suppl):C39-51, 2005. The Fatigue ...

References

{{Reflist, 2 Circadian rhythm