In chronobiology, a circasemidian rhythm is a physiological arousal cycle that peaks twice in a 24-hour day.
Numerous studies have demonstrated that human circadian rhythms in many measures of performance and physiological activity have a 2-peak daily (circasemidian) pattern.
However, a number of published data sets have shown a daily two-peak error pattern in industrial and transportation environments.
Other investigators have reported a circasemidian rhythm in body temperature,[6] melatonin[7] and slow-wave sleep.
[8] 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.