... are specific to living organisms (animals, plants, fungi and some bacteria). They are not quite the same as diurnal cycles, which are directly related to the rotation of the earth and include things like the daily variation in the temperature of the troposphere. These can affect biological processes, but because the latter are not strictly controlled by the earth's rotation (and don't necessarily repeat exactly every 24 hours) they are referred to as circadian – i.e. taking approximately one day to complete a cycle.
© Haydn Thompson 2017