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Why do white blood cell counts change during the day?

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Illustration of a blood vessel with a cutaway view showing red blood cells, white blood cells, platelets, and plasma, each labeled to highlight the main components of blood. — white blood cell Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and water-rich plasma.

In a healthy adult, the number of white blood cells (leukocytes) fluctuates naturally throughout the day. These variations occur on an hourly basis, with the lowest counts typically recorded in the early morning and the highest counts observed in the afternoon.1

Beyond these daily rhythms, white blood cell counts can also change in response to various physiological states. For instance, lower values are generally associated with periods of rest, while higher values are observed during exercise. Other factors that can cause temporary increases in white blood cell counts include pregnancy, menstruation, childbirth, and intense emotional reactions.21

While these fluctuations are normal, significant or persistent changes in the number, appearance, or proportion of white blood cells can be indicative of underlying pathological conditions.1