Why Does Heat Make You Feel Sleepy and Tired?

Science has detected several different reasons why you feel tired after being exposed to heat. They are all tightly connected with homeostasis and the body’s tendency to retain balance. Let’s dig into each cause to gain a better understanding of this phenomenon, and learn what you can do to prevent it from happening.

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Last Updated: Fri, April 21, 2023

Spending a nice sunny day on the beach is what most people would classify as one of their favorite activities. Whether you lie down and bask in the sun, or you spend hours swimming, playing volleyball, or doing something else, it seems that exceptional feelings of sleepiness and tiredness always follow this time. Now it’s clear why that can be the case if you have been engaged in physical activity, but why do you feel so tired after lying around?

And it’s not just the beach. Have you ever listened to a lecture or did something that was supposed to be interesting but you kept catching yourself about to nod off, and the only thing you could concentrate on was not to fall asleep. Maybe it was the middle of winter, and a warm room had that effect of putting you to sleep.

We are sure you found yourself in any of those situations at some point, but you don’t need to worry, as it is entirely normal and science has several explanations to why it happens. Read on to learn more about different causes, and what to do to prevent excessive sleepiness from happening.

What is Homeostasis?

Have you ever wondered how your body always stays around 98.6 degrees Fahrenheit (37 degrees Celsius) even though the outside temperature is drastically different, especially during the winter and summer months? It does so by having multiple thermostats that track your status throughout your whole body. They then send information to your brain, which regulates different processes and keeps your temperature in relatively narrow limits.

Homeostasis is a term that describes a tendency to maintain a stable internal environment within certain parameters, despite external changing conditions. It doesn’t just refer to the temperature regulation, but to all processes in our bodies. For instance, maintaining optimal pH levels in the stomach for better digestion, or maintaining ion concentration, hormone levels, fluid retention, and pretty much everything you can think of. Homeostasis is essential for proper body functioning, and when it’s disrupted, it means that something is seriously wrong and needs to be treated right away. 

Homeostasis maintenance includes many negative feedback loops. They work by perceiving cues, and then sending information to the central nervous system, that then regulates processes that should bring a trigger back to its set point. For instance, when you work out, your muscles produce more heat, and your body temperature naturally rises. Thermostats in your body perceive this change and set information to the part of your brain that should regulate it. Its name is hypothalamus, and it has many essential roles in proper body functioning. The hypothalamus sends a signal to peripheral blood vessels to dilate to promote heat loss and for sweat glands to activate. Both processes are aimed at bringing the temperature down to its set point, and when it happens, a negative feedback loop will stop.

Why Does Heat Make You Feel Tired?

Science has detected several different reasons to why you feel tired after being exposed to heat. They are all tightly connected with homeostasis and the body’s tendency to retain balance. Let’s dig into each cause to gain a better understanding of this phenomenon, and learn what you can do to prevent it from happening.

  1. Heat rises your body temperature.

Your body is a well-oiled system. It gets constant cues from the environment, which are then used to synchronize all processes happening inside you. We’ve mentioned how your body fights to cool down during physical activity, and a similar thing happens when you are hot. Your blood vessels get dilated, allowing more blood to flow near the skin’s surface. This way, warm blood can cool off as it travels near the surface, and it’s also a reason some people appear redder when they are feeling hot. In addition to vasodilatation, sweat glands activate and produce sweat that cools down the skin as it evaporates. This all means that your body needs to put in some extra work to fight excess heat and keep your temperature at optimal levels. That means more energy spent due to increased metabolic and heart rate, which makes you tired in return. And that’s just from being too hot if you combine it with some physical activity, the effects are even stronger. People who carry some extra weight may feel mostly affected by warm temperatures, as larger body means that there is more work to be done to keep everything in balance.

Warm weather also causes a drop in blood pressure. There is less oxygen coming to your brain, which can make you sleepy. Again, your body has to spend extra energy to pump more blood, which can lead to tiredness. You can combat a drop in blood pressure by drinking plenty of water and eating enough salt. You can try crossing your legs while sitting, which has proven to have a quick effect on raising blood pressure.

2. Heat makes you dehydrated.

Most people are always slightly dehydrated as faster lifestyles make us forget to drink water sometimes. When you spend some time in the sun, you lose a considerable amount of fluids through sweating, and your body also uses more water through metabolic processes. These things cause dehydration if you aren’t drinking enough liquid, which usually happens when we are spending time outdoors. 

Besides fluids, you are also losing electrolytes, and if this gets to an extreme, you can get sick, or even die. However, mild dehydration makes you feel sluggish. When there is less fluid, and your electrolyte balance is off, your body needs to work extra hard to maintain everything. Your blood gets thicker as your blood volume is down, which makes your heart pump harder. All of this extra work will make you tired, so be sure to drink plenty of liquids.

There are some common misconceptions about staying hydrated, like that you need to drink at least eight glasses of water each day, or that your urine needs to be transparent; otherwise, you are not doing it right. These are all myths, and the only thing you need to do is listen to your body. It is very well regulated, and nothing says you need to drink water like being thirsty. So forget about stressing if you are drinking 6 or 8 glasses of water each day, your body will tell you exactly how much you need. The only thing that clear urine tells you is that you will probably have to use the restroom again soon.

Summer is a time when you are going to drink more fluids, but be careful of what you drink. Cocktails, wine, and cold beer might seem like the best choice to clench your thirst, but these drinks can make you more dehydrated. Alcohol suppresses the production of vasopressin, a chemical that is essential to retaining body fluids. When there is not enough vasopressin, you will excrete more urine, which perfectly explains all the restroom visits after having a couple of beers. That’s why you should always make sure to drink responsibly, and take water in between each round.

3. Excessive sunlight exposure can harm your body.

Sun doesn’t only raise your temperature. Its ultraviolet rays can cause cell damage, pigmentation changes, sunburn, and wrinkles. Prolonged exposure to ultraviolet light drastically increases the chances of developing skin cancer, so make sure to avoid going out during the hottest parts of summer days. Also always wear sunscreen to protect yourself from these harmful effects.

With more time spent in the sun, your body will have more work to do repairing the damage, which often leads to fatigue. For instance, when you get sunburned, your body moves more fluid to the area that has been damaged in an attempt to address the issue. However, that means there is less fluid to sweat, which reduces the body’s ability to cool itself down. That makes you dehydrated, and you quickly become fatigued. 

If you spend too much time outside, especially if doing some physical activity, you might be at risk of heat exhaustion and stroke. In most cases, you just need to get away from the sun and be somewhere cooler, but if along with fatigue you start experiencing heavy sweating, dizziness, lightheadedness, headache, nausea, or unusually weak or rapid pulse, you should seek medical help right away. Heat stroke is a serious condition that requires immediate emergency treatment, as it can cause permanent damage to the brain, kidneys, heart, and muscles. Other symptoms include agitation, confusion, slurred speech, or seizures.

4. Heat can trick your brain.

Your body has an internal clock that regulates everything you do, like when is the time to eat, be active, or go to sleep. These patterns are called circadian rhythms, and they are characteristic of all living things. One of the ways to synchronize this internal clock with the environment is through our senses. Our brain perceives different stimuli, like temperature and light, and regulates processes in our body accordingly.

Melatonin is a hormone responsible for regulating our sleep and wake cycles. Its production usually starts in the evening, due to the natural drop in temperature and decrease of light. These are the signals that when our brain recognizes, it starts secreting melatonin, and we become sleepy. Levels of this hormone are high through the night to maintain sound sleep, and then significantly drop in the morning, when we are supposed to be wide awake and active.

When we are outside in the bright light, it can restart our inner clock. Melatonin production is suppressed in these conditions, and we don’t feel sleepy at all. However, fatigue can kick in when we get back home and enter an environment where there isn’t much bright light, and the temperature is much cooler. That can trick our brain into thinking that it’s night, and it can start producing melatonin, making us feel sleepy and tired in return.

Behavioral Links Between Heat and Sleepiness

Excessive heat doesn’t only have physiological effects on your body; there are also some connections your brain has made with feeling warm and certain behaviors you usually do in these conditions. For instance, we typically associate being cozy and warm under a blanket with sleepiness. That can mean that we instantly get sleepy when we feel the heat, as our brain has made that connections. You can combat this by learning new associations to heat, such as doing your favorite activities that make you excited and alerted. 

Besides obvious reasons for feeling tired during summer, there are a few ones that are often overlooked. Summertime can bring a lot of stress into our life as kids are on a break, so there is no school, and they can require constant care and attention which can be exhausting. Planning family holidays, making sure that everybody is ready and packed also adds to stress, as does the general change in routine that summertime brings. When we are worried, our metabolism isn’t as efficient as usual, and our bodies also spend more energy preparing to deal with a stressful situation. All of this leads to increased feelings of fatigue and sleepiness, so if you are feeling exceptionally distraught, relaxing can help you regain your energy levels.

It’s surprising how many people forget that tiredness is mostly a sign of the lack of sleep. Many people struggle to fall and stay asleep during summer nights, as warm temperatures make it harder to relax since you are sweating and unable to get comfortable. You can try doing several things to help you sleep in these conditions:

  1. Maintain a steady sleeping routine. That way, your brain will know when to rest, and it should make it easier to fall asleep. Keeping a consistent schedule should also allow you to getter deeper restorative sleep.
  2. Choose the coldest place in your house for sleeping. It should also be dark and quiet to remove distractions and make falling asleep easier. You should keep an open door or window, to allow proper air circulation, and you can even use fans for this. 
  3. Make sure you are drinking enough water throughout the day, as your body is better at regulating temperature when it is fully hydrated. You should drink a glass of water before going to bed, but don’t overdo it as you don’t want to wake up in the middle of the night to run to the bathroom.
  4. Eat a balanced diet with plenty of fruits, vegetables, whole grains, and lean protein. Avoid having large and spicy meals before bedtime, as well as chocolate and caffeinated drinks, as they can keep you up and make it harder to fall asleep. 
  5. Stay active but avoid exercising too close to bedtime to give your body enough time to cool down properly. 
  6. Consider getting a mattress with good temperature neutrality, such as innerspring or latex.
  7. Wear loose clothes made from natural materials like cotton or rayon. These are highly breathable; they absorb sweat and allow your body to breathe. Your bedding should also match. You can even consider sleeping naked, as it has various benefits, including excellent thermoregulation.
  8. Take a cold shower before bed as that will help cool your body down. You can also keep a bowl with water or a damp towel next to your bed if you wake up feeling hot.

How to Avoid Heat Fatigue

If you feel exceptionally sleepy during summertime, but you want to be more energized to fully enjoy different activities, we have some tips on how to do that. They should prevent fatigue and other consequences that are linked with spending too much time in the sun.

  1. Limit your exposure to direct sunlight. Try to stay inside during the hottest parts of the day, and when going out, stay in the shade as much as possible. That way you can enjoy the benefits of time spent outdoors without getting too hot, sunburned, and you’ll lose less liquid. It is essential to prepare before going out. Be sure to always put on sunscreen to any exposed area of your body. Wear hats, sunglasses, and highly breathable long sleeves to prevent sun damage.
  2. Stay hydrated. The best way to combat dehydration is to drink plenty of water. Always bring a bottle with you and know where your nearest drinking fountains are. Eat a lot of fruits and vegetables as they also have high water content. Remember that drinks that contain alcohol act as diuretics, and they will increase your dehydration levels. 
  3. Always have a salty snack nearby. When you sweat, you are also losing salt and other electrolytes beside water. You should increase your salt intake, and bring a salty snack like nuts, pretzels, or baked potato chips with you when you go outside.
  4. Know when to take a break and get away from the sun. If you are partaking in physical activity, know your limits. When you feel like it’s too much, step aside, rest for a bit, and don’t let your body go into overdrive.


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Dusan is a biologist, a science enthusiast and a huge nature lover. He loves to keep up to date with all the new research and write accurate science-based articles. When he’s not writing or reading, you can find him in the kitchen, trying out new delicious recipes; out in the wild, enjoying the nature or sleeping in his bed.

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