A blood test taken in the morning could give subtly different results from one done in the evening, or at a different time of year. The discovery may affect whether people are diagnosed with conditions like anaemia or deemed eligible to donate blood. Anaemia occurs when the number of red blood cells, which contain the oxygen-carrying protein haemoglobin, is lower than normal or if they have too little haemoglobin.
Cases are often missed, while low levels of iron or haemoglobin can affect whether someone is judged healthy enough to give blood. How to harness your body clock for a longer, healthier life “The differences [in the study] are small, but they could matter for people whose haemoglobin sits right on a diagnostic or blood-donation threshold,” says Tami Martino at the University of Guelph in Canada, who wasn’t involved in the research. For instance, red blood cells show daily changes in the chemical state of haemoglobin, which may influence signals that dilate blood vessels near the skin, contributing to fluctuations in body temperature.
In the latest study, Sophie Wehrens at Sanquin Blood Supply Foundation in Amsterdam, the Netherlands, focused on a different aspect of blood biology: how concentrations of haemoglobin and an iron-storage protein called ferritin vary at different times of day and over the course of the year. The researchers analysed blood measurements collected from around half a million Dutch people when they registered as donors between 2013 and 2023. They found that haemoglobin and ferritin levels vary with the seasons, peaking in December and reaching their lowest levels around June to August.
This could be due to the body’s response to warmer temperatures, says team member Katja van den Hurk, also at the Sanquin Blood Supply Foundation. When it is hot, the body directs more blood towards the skin to help shed heat, while prolonged exposure to warmer temperatures can increase the volume of blood plasma, the liquid portion of blood. This then increases the volume of circulating blood, which lowers the concentration of haemoglobin and other blood biomarkers like ferritin, says van den Hurk.
The researchers also found that haemoglobin levels followed a clear daily pattern, peaking at around 10.30am and gradually falling as evening approached. The size of this fluctuation was slightly greater in men than women, and in both sexes, the daily peak occurred around an hour earlier in people aged over 45. The age finding fits with what we already know about how our circadian rhythm changes throughout our life, says Martino.
The circadian rhythm describes the natural oscillations in the activity of our tissues, driven by an internal clock over a 24-hour period. For instance, teenagers and young adults are generally more like night owls and become more lark-like as they get older, says Martino. The sex differences could partly reflect the influence of sex hormones, she says, but the study didn’t look into this.
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