Circadian rhythms in parasites

Not all daily rhythms are circadian rhythms. The earth’s rotation exposes organisms to rhythmic changes in the environment, such as light/dark cycles. These environmental changes impose rhythms in organisms; however, many of these rhythms may not persist when in constant conditions. The 24-hour rhythms that do persist in these constant conditions are circadian rhythms. For a process to be defined as a circadian rhythm, it must fulfill 3 properties: i) have a periodicity of around 24 hours (± 2 hours), even in the absence of environmental cues (free-running period); ii) be able to readjust its timing (phase and period) in response to environmental cues (entrainment); iii) and maintain an approximately 24 hour period at different temperatures within the organism’s physiological range (temperature compensation of the period) (Fig 1A–1C).