Did you know that your body temperature normally fluctuates up to 1°C (~1.8°F) in a day? But why?\
THE DAILY CORE TEMPERATURE CYCLE
Your core temperature is lowest around 4-6 AM and highest around 4-7 PM. Biological circadian rhythms are ~25 h in length, and so the time of day these peaks and troughs occur will slightly shift each day.
This is important to remember when working in the heat, because your core temperature will always be lower in the morning than in the afternoon, and so it is important to understand whether your core temperature is increasing as a result of heat stress or just due to typical fluctuations in body temperature throughout the day.
WHAT CAUSES THIS CORE TEMPERATURE FLUCTUATION?
The short answer: melatonin. Melatonin is a hormone that has a tight control on your body temperature: when your body increases melatonin at night, this leads to a decrease in your body temperature (by ~0.3°C) and causes you to get sleepy. This is one of the reasons that taking melatonin might help you fall asleep.
However, studies have shown that melatonin does not reduce your body temperature enough to help you stay cooler in the heat.
HOW DOES THIS CORE TEMPERATURE FLUCTUATION AFFECT YOUR ABILITY TO WORK IN THE HEAT?
Although your ability to dissipate heat is just as good in the morning as in the afternoon, the slightly higher core temperature in the afternoon can reduce your productivity because you might achieve a higher core temperature sooner. That means you might need to take more breaks to stay cool in the afternoon vs. the morning work shifts.
One solution is to try to get all of your hard work done in the morning when it’s cooler outside and your body temperature is lower, and to save the smaller, lighter work tasks for the afternoon.
Have more questions? Kenzen is here to help with our Heat Safety Training Program.
1) Racinais, S. “Different effects of heat exposure upon exercise performance in the morning and afternoon.” Scandinavian journal of medicine & science in sports 20 (2010): 80-89.
2) Cheung, Stephen S. Advanced environmental exercise physiology. Human Kinetics Publishers, 2009.