Accurately determining core temperature is critical in the treatment of COVID-19 patients. However we should not forget that the core temperature is anything but static. After all, the circadian rhythm – also called the Circadian Cycle – means that a patient’s temperature can vary up to 0.9 oC during the day. At the same time this slightly higher temperature does not automatically mean this person has an elevation or even a fever.

Fever is an increase in a person’s normal average temperature. In an absolute sense the difference between an elevated temperature and actual fever is small. When measuring patient after patient a deviation of 0.9 oC can occur all too easily. Such a deviation can be very determining for the treatment method.

Important factors

Measuring the core temperature of a patient must therefore be done very accurately. 3 factors play an important role here:

  • The measuring method of the thermometer.
  • The care with which the measurement is carried out.
  • The interpretation of the measured core temperature.

Measurement method

First of all, the applied measuring method and its accuracy. The question is which characteristics of the measurement method we consider important here. For example, if we compare rectal and temporal measurements – where the temperature is measured on the temporal artery on the forehead – we see that the accuracy is basically the same. However, if we take into account factors such as ease of use and comfort, we see that the rectal measurement does have some drawbacks compared to non-invasive measurement methods such as temporal. Ear measurements are popular, but have the disadvantage that the accuracy can be strongly influenced if the measurement sensor of the thermometer is not correctly positioned in the ear canal.


This brings us to the interpretation of the measured temperature. Many thermometers are capable of accurately determining a patient’s body temperature. At the same time we know that a person’s normal body temperature varies throughout the day – which is what we call the circadian rhythm. The temperature at certain times during the day varies from person to person and is based, among other things, on external temperatures, age, gender, activity level and, above all, the time of day. The body temperature can also change when someone is cold, hungry or sleepy. This variation can be as high as 0.9 oC.

Now suppose that a measurement shows a temperature of 39.1oC. Doctors and nurses are fully aware of the fact that some thermometers are more accurate than other measurement methods. For example, we know from ear thermometers that measurements can deviate significantly from the actual core temperature. This deviation is considerably smaller with temporal artery measurements. The actual core temperature of the person is therefore within a specific bandwidth with respect to the measured value.

Circadian rhythm

In order to carefully determine whether a patient has a fever, we have to take into account the circadian rhythm of the person. The problem here is that we do not know a person’s circadian rhythm. Recently Exergen has developed a procedure – for which a patent has now been granted – in which a series of historical measurement values ​​is built up. These measurements take place during the day and over several days. We need at least two measurements: one early in the morning (7 AM) and one measurement at the end of the working day (6 pm). This gives us a series of body temperatures spread over the day for each patient. This gives us a much better insight into the variation in the body temperature of a patient.

The moment we now take a new measurement of the core temperature of a patient we can compare the measured value with the results of previous measurements that were recorded at more or less the same time. Obviously, this does not necessarily make the measurement of the temperature itself more accurate. But it does help us to come to a much better interpretation of the measured core temperature.

Don’t forget to measure yourself based on circadian rhythm

Doctors and nurses are at a high risk in this pandemic. So it is very important that they measure themselves regularly. However, the circadian rhythm discussed in this article means that they must be careful in interpreting their own body temperature measurements. So here is our advise: as a doctor or nurse please record the measured values several times during the day (at least 7 AM in the morning and 6 PM in the evening) and repeat these measurements for several days. This creates a good picture of your own circadian rhythm. This overview of data points provides a much better basis to determine whether you have a raised temperature or even a fever.

Recently Published Studies on Circadian Effects on Fever

Want to read more about circadian rhythm? Here are 3 interesting studies:

  • Harding et al (2020). Fevers Are Rarest in the Morning: Could We Be Missing Infectious Disease Cases by Screening for Fever Then? Undergoing peer review at
  • Harding et al (2020). Fever incidence Is Much Lower in the Morning than the Evening: Boston and US National Triage Data. West J Emerg Med. 2020 Jun 24;21(4):909-917. doi: 10.5811/westjem.2020.3.45215.
  • Harding et al (2019). The daily, weekly, and seasonal cycles of body temperature analyzed at large scale. Chronobiol Int. 2019 Dec;36(12):1646-1657. doi: 10.1080/07420528.2019.1663863. Epub 2019 Sep 17.

Here is another valuable source of information:


Dr. Joaquin Azpilicueta MD is Director of Sales, Exergen Corporation