Circasemidian rhythm

In chronobiology, 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 in many measures of performance and physiological activity have a 2-peak daily (circasemidian) pattern. The word, circasemidian, is based upon the Latin 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 in body temperature 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 accepted internal timing source for the major circadian rhythms of the body. Thus, though it falls in the domain of an ultradian rhythm, the circasemidian rhythm may be the first harmonic 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. Bjerner B, Holm A, Swennson A. "Diurnal variation in mental performance: A study of three-shift workers". Br J Ind Med 12:103-110, 1955. - Browne RC. "The day and night performance of teleprinter switchboard operators". Occupational Psychology 23:1-6, 1949. - Folkard S, Lombardi DA, Tucker PT. "Shiftwork: safety, sleepiness and sleep". Industrial Health 43:20–23, 2005. - Harris W. "Fatigue, circadian rhythm, and truck accidents". Chapter 8 in Mackie RR (ed.), Vigilance: Operational Performance, and Physiological Correlates. New York, Plenum Press, 1977. - Harris W. "Relationships between length of time driving, time of day, and certains kinds of accidents". Pages 51-64 in Mackie RR, Miller JC, Effects of Hours of Service, Regularity of Schedules, and Cargo Loading on Truck and Bus Driver Fatigue (NTIS PB290957). Goleta CA, Human Factors Research, Inc., 1978. - Hildebrandt G, Rohmert W, Rutenfranz J "12 and 24 h rhythms in error frequency of locomotive drivers and the influence of tiredness". Int J Chronobiology 2:175-180, 1974. - Kogi K, Ohta T. "Incidence of near accidental drowsing in locomotive driving during a period of rotation". J Human Ergology 4:65-76, 1975. - Langlois PH, Smolensky MH, His BP, Weir FW. "Temporal patterns of reported single-vehicle car and truck accidents in Texas, USA, during 1980-1983". Chronobiologia 2:131-140, 1985. - Lavie P, Wollma M, Pollack I. "Frequency of sleep–related traffic accidents and hour of the day". Sleep Research 15:275, 1986. - Mitler MM. "Two-peak patterns in sleep, mortality and error". Proc Intl Sympos on Sleep and Health Risk, Springer Verlag, 1989. - Prokop O, Prokop L. "Ermuding und einschlafen am steuer". Dtsch Z Gerichtl Med 44:343-355, 1955. 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.

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 modeling efforts such as the Fatigue Avoidance Scheduling Tool.