Diurnal Variation

Diurnal variation is the phenomenon where hormones fluctuate throughout the day and night. For example, cortisol levels are normally higher in the morning and lower at night. It can vary throughout the day depending on exercise or stress. This cycle makes sense for cortisol to be high in the morning and low at night—as the day begins, humans would like to be as awake and alert as possible. Similarly, when trying to sleep, cortisol levels are lower so that you don't feel anxiety while trying to sleep.