diurnal variation

Diurnal variation is the predictable rise and fall of a hormone's level over the course of a day and night. If circadian rhythm is the underlying internal clock, diurnal variation is what you actually measure in the blood at different hours — a hormone reading that means something quite different at 8 a.m. than at midnight.

Cortisol is the textbook case: it is highest in the early morning, around the time of waking, and lowest late at night during early sleep. Because of this, the time of day a blood sample is drawn must be recorded for cortisol to be interpretable, and a midnight value that fails to fall to its normal trough is a key clue to cortisol excess.

In strict usage diurnal can imply daytime specifically, but in clinical endocrinology diurnal variation is used broadly for any consistent day-versus-night pattern. The practical lesson is the same: many hormone reference ranges are time-dependent, so a single number without a timestamp can be misleading.