CORE TEMPERATURE & CIRCADIAN RHYTHM

CORE TEMPERATURE & CIRCADIAN RHYTHM

clock with human hands around it

Did you know that your body temperature normally fluctuates up to 1°C (~1.8°F) in a day? But why?\

Photo by Oladimeji Ajegbile on Unsplash

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.

construction site

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.

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REFERENCES:

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.

WHEN YOU’RE TOO HOT: FEVER VS. HEAT ILLNESS. WHAT’S THE DIFFERENCE?

WHEN YOU’RE TOO HOT: FEVER VS. HEAT ILLNESS. WHAT’S THE DIFFERENCE?

Both infection and heat stress can lead to dangerously high core body temperatures.

But how can you tell the difference these 2 things physiologically, and should they be treated differently?

thermometer

With the rapid spread of the coronavirus (SARS-CoV-2/COVID-19), the Kenzen team wanted to make sure that anyone who might be continuing to work (or live) in hot climates is aware of the the differences between heat-related illnesses (e.g., exertional heat exhaustion or heat stroke) vs. fever, so that you can stay safe.

Body Temperature Regulation: The Basics

Your body wants to maintain it’s temperature around a ‘“set-point” where it’s most comfortable and works most efficiently. This set-point is typically around 97.9 to 98.8°F (~36.6-37.1°C). Of course, this set-point differs for each individual— based on how fit they are, whether they are heat acclimatized, and what time of day it is— but in general, your body tries to maintain its temperature within this narrow range.

This set-point is controlled by a region in your brain called the hypothalamus, where a group of cells (neurons) sense changes in your body’s temperature and then send out responses to adjust the temperature accordingly. There are also neurons in your skin that sense hot and cold, and then relay this information to the brain. Your brain combines all of this information together and if your temperature is outside of that set-point, it will cause your body to respond by either increasing (e.g., through shivering) or decreasing (e.g., through sweating) your temperature to get it back to normal.

brain

So what happens with a fever?

  • A fever just indicates that your body temperature is higher than normal. This is typically above 100.4°F (~38°C) and occurs from an infection (either viral or bacterial) that is causes substances, called pyrogens, to leak outside of the invader cells.

  • These pyrogens increase the body’s set-point, which means that now instead of having a set-point of say 97.9°F, your new set-point is 103°F.

  • Because you have a new (higher) set-point, your body now thinks that a temperature of 97.9°F is “cold”, and so your body will try to increase your temperature.

  • This causes the typical responses when you’re cold, like 1) shivering (chills), 2) vasoconstriction (narrowing) of the vessels in your skin which makes your skin colder because there is less blood flowing to the skin, and 3) increased cellular metabolic heat production.

  • All of these responses cause your body temperature to go up. It continues to go up (in some cases for hours) until your temperature hits this new set-point of 103°F. At this point, you might feel OK because you’re at your “new normal.” However, as soon as you cross this point, or you take medications that inhibit the pyrogens, your body set-point goes back down, and now your body temperature is way too high.

  • What does your body do? That’s right, it tries to cool you down by sweating, vasodilation, etc… which is when you feel really hot and sweaty.

  • This cycle of shivering and sweating can continue for as long as you are fighting the infection.

How do you know if you have a fever?

If you ARE NOT working/exercising in the heat, and you feel chills or cold skin, have body aches, and/or feel weak & tired, you may have an infection that should be treated by a medical professional.

Note: If you ARE working in a hot environment, and your core temperature does not feel like it has been increasing steadily, you are intermittently getting chills, and/or you feel cold or achy, you might have something that is unrelated to heat stress.

What’s the best way to treat a fever?

Contact your doctor, stay home, drink plenty of fluids, and rest.

construction workers

What happens with heat-illnesses (like heat stroke)?

  • Unlike with a fever (from an infection), when you have a heat-related injury or illness, your body temperature (hypothalamic) set-point has not changed.

  • With heat stress, your core body temperature continues to increase past your set-point because your metabolic heat production (from working or exercising really hard in the heat) is exceeding your body’s ability to get rid of the heat you’re generating.

  • And once your core body temperature goes up past the set-point, then you start sweating to try to cool your body down. But remember, if it’s really humid and hot outside, then it won’t be as easy for you to dissipate that heat (because the sweat won’t evaporate).

  • So if you’re working hard in the heat, and you are producing heat faster than you can get rid of it, your body temperature will continue to climb.

  • Often, while working or exercising in the heat, core body temperatures can easily (& safely) exceed the “fever” criteria of 100.4°F. In fact, reaching this core body temperature is often necessary to acclimatize to the heat.

  • Most trained athletes & heat-acclimatized workers can safely reach and maintain core body temperatures of 101.3°F (38.5°C) without any damage to their body.

  • However, the main difference (between reaching this higher core temperature with heat stress vs. fever) is that this is a safe increase in core temperature that has not altered the brain’s set-point. And furthermore, your body is not too hot where you could get irreversible (organ) damage.

  • That being said, if people are not closely monitored for signs & symptoms of heat-related injuries & illnesses, these hotter temperatures can lead to heat stroke and even death.

How do you know if you have a heat-related illness?

Signs & symptoms can vary, but some of the first signs of heat exhaustion are really hot skin, very sweaty, feeling lightheaded or faint, difficulty continuing to work/exercise, and general weakness.

Exertional heat stroke means that your core body temperature is >104°F (40°C) and it is a serious medical emergency. Along with this extremely high core body temperature, the person will often exhibit changes in their behavior (e.g., aggressiveness, confusion, irritability), can collapse or faint, and is very weak.

The main point is that you will generally be feeling hot, sweaty, & faint with heat-related illness, more so than cold, chilly, & achy, like with a fever.

What’s the best way to treat heat-related illnesses?

For heat exhaustion: rest (for at least 15 min), drink water, and find shade or air conditioning. Continue to monitor your symptoms & if possible, track your body temperature throughout the day.

For exertional heat stroke: emergency cooling using an ice water bath is best, & call emergency medical services immediately. Remember: cool first at the site, and then transport to the hospital.

Stay safe & healthy, everyone!

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REFERENCE:

Guyton, Arthur C., and John Edward Hall. Textbook of medical physiology. 12th edition. Philadelphia: Saunders, 2011.

DOES POURING WATER ON YOUR HEAD ACTUALLY COOL YOU DOWN?

DOES POURING WATER ON YOUR HEAD ACTUALLY COOL YOU DOWN?

We’ve probably all been there: sweating it out in the heat, wishing we were in a cool lake or pool, and so we reach for that water bottle and pour the water on our head for some relief.

But does this actually cool you off or is this relief just temporary?

underwater

The answer: it depends on your conditions.

In a hot-dry environment (low humidity):

  • Pouring water over your head can cool you off and lower your core body temperature, but the caveat to this is that you have to be in a place where the water you poured on your body can evaporate off of your skin and get rid of heat (remember, only sweat that evaporates off of your skin actually cools you off).
  • A good breeze is also important to help wick the water off of your skin to cool you down.
    And as you’ve probably guessed, you’d also need to be wearing minimal clothing— like shorts & a t-shirt— as heavy clothing, like PPE, would just trap the water you poured on your body.

strong>In a hot-wet environment (high humidity):

  • If your sweat is already dripping off of you and it’s really humid outside then pouring water over your head might make you feel better, but won’t help you cool down.
  • Research has shown that pouring water over your head can make you feel cooler by reducing your skin temperatures and also reduce your perceived exertion (how hard you feel like you’re working) in the heat. However, it won’t improve your performance.

So, what’s the verdict on pouring water over your head?

Unless you’re wearing light clothing and working in a hot-dry environment where the water can evaporate off of your skin, it won’t help to cool you off. That being said, if it makes you feel better to pour cold water over your head and helps you get through the work day, it won’t hurt you, so go for it- just remember to keep an eye on the signs & symptoms of heat illness.

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REFERENCES:
Munoz, C. X., et al. “Effects of oral rehydration and external cooling on physiology, perception, and performance in hot, dry climates.” Scandinavian journal of medicine & science in sports 22.6 (2012): e115-e124.
Morris, Nathan B., and Ollie Jay. “To drink or to pour: How should athletes use water to cool themselves?.” (2016): 191-194.

WHAT IS YOUR SWEAT RATE & WHY DOES IT MATTER?

WHAT IS YOUR SWEAT RATE & WHY DOES IT MATTER?

Calculating your sweat rate is the best way to figure out how much water you need to be drinking when working or exercising in the heat.

You can use our sweat rate calculator (see below) to figure out your average sweat rate, and how much water you should drink back after your exercise bout or work day.

WHAT YOU NEED TO DO TO GET AN ACCURATE SWEAT RATE CALCULATION:

  1. Pick an activity (ideally during your typical workday) that is 30-60 minutes long, where you are usually sweating. Note that you will need to weigh yourself (nude) before and after the activity, so if this is not possible at work, then go for a run at home or the gym, or do some sort of aerobic (endurance) activity that gets your heart rate up & causes you to sweat (e.g., elliptical, cycling, etc).
  2. Weigh yourself (nude) before exercise and/or work. Write down this number. Go do your activity – and be sure to keep track of the time (be as precise as possible). Note: During this exercise period, you should not use the restroom or drink any fluids. Immediately after you’re done, take your clothes off & wipe off any dripping sweat from your body.
  3. Weigh yourself (nude) again. Write down this number. Enter your two body weights & your exercise/work duration in the calculator below to get your sweat rate & how much water you should drink back!

Sweat Rate Calculator

 

Note that the amount of water to drink back is the amount specific to the activity you did (and that amount of time). If it’s really hot outside, your sweat rate might be even higher, and then you’ll need to drink even more water.

**Remember that you can’t just chug all of that water at once because your body can only absorb it so fast.. as a general rule of thumb, your body can only absorb ~1 cup (8 oz.) of water every 15 minutes, so try to divide up your fluids (over the hour) based on that rule.

For more information, check out our blog on staying hydrated during the workday.

THIS IS A GOOD PLACE TO START, BUT FOR MORE CUSTOMIZED HYDRATION PLANS & RECOMMENDATIONS SPECIFIC TO YOUR SITE, WORK ACTIVITIES, AND CLOTHING, CHECK OUT KENZEN’S HEAT SAFETY PROGRAM– WE’RE HERE TO HELP.

HEAT ACCLIMATIZATION: WHAT IS IT AND WHY DOES IT MATTER?

HEAT ACCLIMATIZATION: WHAT IS IT AND WHY DOES IT MATTER?

Most heat-related deaths occur in the first few days on the job. Heat acclimatization can help prevent this.

A study of OSHA citations issued between 2012 and 2013 revealed 20 cases of heat-related illness or death of workers [Arbury et al. 2014]. In most of these cases, employers had no program to prevent heat illness, or programs were deficient; and acclimatization was the program element most commonly missing and most clearly associated with worker death.

How Do You Acclimatize to the Heat?

You might have already heard about heat acclimatization, but what is it and why does it actually make your job easier?

Heat acclimatization is a process where your body undergoes physiological changes that allow it better handle the heat— which means that your performance in the heat will improve and you are less likely to suffer from heat-related problems (e.g., heat cramps).

For heat acclimatization to occur, you typically have to increase your core body temperature by 1°C (~1.8°F) for at least 1 hour, repeatedly each day, for 5-14 days in a row (each person will take a different amount of time to acclimatize to the heat).

Physiological Changes that Occur with Heat Acclimatization

After your body is acclimatized to the heat, you will notice lots of different changes.

  1. Sweating: You will start to sweat sooner (at a lower core body temperature) and you will sweat more. This is important because it allows your body to cool off (through evaporative heat loss), which means that your body temperature will stay lower for the same work rate. (See a refresher on heat loss through sweating here). You may also notice that your sweat becomes less salty after you’ve been working in the heat for a while; this is because your body becomes more efficient at reabsorbing the salt in your sweat, which helps you stay better hydrated in the heat.
  2. Heart rate: Your heart rate at rest and during work (or exercise) will be lower after heat acclimatization. This is because you get an increase in plasma volume (the water in your blood) with heat acclimatization, which means that with each heart beat, you can now pump more blood (to get more oxygen to the working muscles). As a result, your heart rate will be lower for the same work rate. This means that you can work for longer in the heat without getting tired. (See a refresher here on heart rate changes with heat stress).
  3. Core and skin temperatures: Due to the improvements in your sweating (and improved ability to get rid of heat), your core and skin temperatures will be lower at rest and during work or exercise (for the same work rate). This, combined with a lower heart rate, means that after heat acclimatization, you can work harder for longer periods of time before needing to take a break because your body is staying cooler.

worker in hot field

Important Tips During Heat Acclimatization:

  • Make sure to drink plenty of fluids before, during, and after the heat acclimatization period (your first two weeks on the job), and replace the water that you’re losing through sweating. This is especially important for helping your body to increase its plasma volume.
  • During heat acclimatization, you can lightly salt your foods as well as drink fluids with more sodium to help you stay hydrated and maintain your body’s electrolyte balance.
  • You can monitor your heart rate at the same time each morning before work (by taking your pulse) during heat acclimatization. You should see a decrease in your resting heart rate over the course of 1-2 weeks, which will indicate that you have likely acclimatized to the heat.
  • You should acclimatize to the environment that you will be working in. The changes that your body makes to a hot, dry environment will be different than those of a hot, humid environment.
  • Be aware of any signs & symptoms of heat injury & illness, and do not push yourself too hard the first week that you are just starting to work in the heat. This process takes time for your body to adjust, and trying to go too hard too fast will only result in serious accidents.
  • If you work in an environment with heavy clothing or PPE, if possible, you should start by wearing the most minimal clothing layer possible on the first 1-2 days, and then slowly adding each additional clothing layer (every 1-2 days) to give your body time to adjust over the course of the 1-2 week heat acclimatization period. As a rule of thumb: you can wear full PPE from day 6 onwards.
  • Lastly, remember that to maintain your heat acclimatization, you’ll need to be exposed to the heat at least once every 5 days. (See here for more info on the decay of heat acclimatization).

Need help figuring all of this out for your workers & work-site? Kenzen can help with our site-specific evaluations, heat safety training, and individualized. recommendations

HEAT ACCLIMATIZATION IS THE BEST “PERFORMANCE ENHANCEMENT” OUT THERE (FOR WORK IN THE HEAT) BECAUSE IT IMPROVES YOUR PERFORMANCE AND KEEPS YOU SAFER— SO MAKE SURE THAT YOU (OR YOUR WORKERS) ARE HEAT ACCLIMATIZED!

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References:

Pryor, J. Luke, Christopher T. Minson, and Michael S. Ferrara. “Heat acclimation.” Sport and Physical Activity in the Heat. Springer, Cham, 2018. 33-58.

Coco, Aitor, Brenda Jacklitsch, Jon Williams, Jung-Hyun Kim, Kristin Musolin, and Nina Turner. “Criteria for a recommended standard: occupational exposure to heat and hot environments.” control Ccfd, editor (2016).