The human body undergoes a series of physiological adjustments throughout the day, which include the regulation of body temperature. These adjustments are mainly controlled by neurons in the suprachiasmatic nucleus, located in the anterior hypothalamus, responsible for the functional oscillations observed in various organs and tissues. These oscillations affect not only the sleep-wake cycle but also respiration, heart rate, blood pressure, peristalsis, motor system, mental functions, nervous activity, and body temperature (Aschoff, 1983).

To correctly interpret body temperature, it is essential to consider its normal fluctuations throughout the day. These daily variations have been demonstrated by measuring rectal, axillary, gastrointestinal, oral, and skin temperatures. In individuals with normal lifestyles, rectal temperature shows the highest values between 2:00 PM and 8:00 PM, with a peak around 5:00 PM and a minimum in the early morning (Edwards et al., 2008).

We find some research works like Costa, C. M. A et al., (2016) on Daily Oscillations of Skin Temperature in Military Personnel Using Thermography, which we address below.

Skin Temperature in Military Personnel Using Thermography
The objective of this research by Costa and collaborators was to determine the oscillations in skin temperature in 25 regions of interest (ROIs) of the body throughout a day. This study was conducted with 31 military personnel from the Brazilian Air Force (Age: 22.9 ± 3 years, Height: 178.3 ± 7.8 cm, Body Mass: 73.4 ± 8.2 kg, Body Composition: 9.6 ± 3.5%), evaluating thermal variations at different times of the day.
Thermographic images were taken at five different times of the day: 7:00 AM, 11:00 AM, 3:00 PM, 7:00 PM, and 11:00 PM. During breaks, the subjects performed sedentary activities with low energy consumption to minimize any thermogenic effect. Data collection was carried out over two days in the spring season, maintaining an ambient temperature of 25°C. The subjects did not perform intense physical exercise the day before the evaluation and followed a normal daily routine. An IRT-25 camera (Fluke, Everett, USA) was used, and the images were analyzed using software.

Main Results
The anterior hands showed the greatest temperature variations throughout the day.
ROIs in the lower extremities, scapula, abdomen, chest, and lower back showed significant differences between the 7:00 AM measurements and other times of the day.
The lowest temperature values were recorded early in the morning (7:00 AM).
There was an increase in temperature during the afternoon, stabilizing after 3:00 PM.
All ROIs and the average skin temperature showed oscillations throughout the day.
The hands showed an upward central trend throughout the day.
Temperature differences depended on the specific body region.
Distal extremities showed greater thermal oscillations.
The study revealed that skin temperature in all ROIs showed oscillations throughout the day, with the lowest values in the early morning (7:00 AM). Temperature fluctuations depended on the specific body region, with thermal stabilization in some regions in the afternoon and an upward trend in the hands during the day.

Conclusions
Below we highlight the most relevant conclusions of Costa and collaborators’ study:

All body regions studied showed temperature variations throughout the day. Distal regions (hands, feet) had greater variation.
The lowest temperatures were recorded in the morning and the highest in the evening.
Thermography allows creating a map with normal thermal characteristics for each body region.
Therefore, from Thermohuman, we point out that infrared thermography is a very versatile tool for various applications in the military field. It allows detecting and monitoring changes in body temperature non-invasively and in real time. Some of the main uses of thermography applied to military personnel, police, and other security forces are:

Injury prevention: Monitoring skin temperature can prevent heat-related injuries, such as heatstroke and dehydration, and cold injuries, such as frostbite.
Fatigue detection: Changes in skin temperature can indicate fatigue levels, allowing adjustments in activities and rest times to optimize performance and recovery process.
Training optimization: By monitoring the body’s thermal responses during training, more efficient and safer exercise programs can be designed, tailored to each individual’s physiological conditions.
Injury follow up and recovery: Allows monitoring the progress of injury rehabilitation, evaluating treatment effectiveness, and adjusting therapies as necessary.
Referencias
Aschoff, J. (1983). Circadian control of body temperature. Journal of thermal Biology, 8(1), 143-147.

Costa, C. M. A., Sillero-Quintana, M., Cano, S. P., Moreira, D. G., Brito, C. J., Fernandes, A. A., … & Marins, J. C. B. (2016). Daily oscillations of skin temperature in military personnel using thermography. BMJ Military Health, 162(5), 335-342.

Edwards, B., Waterhouse, J., & Reilly, T. (2008). Circadian rhythms and their association with body temperature and time awake when performing a simple task with the dominant and non‐dominant hand. Chronobiology international, 25(1), 115-132.