Why Does the Sun Make Me Tired?

10 Min Read | By Liam Porter

Last Modified 3 March 2021   First Added 21 September 2020

This article was written and reviewed in line with our editorial policy.

Whether it’s a vacation, staycation, or a warm summer’s day, you’ll likely have noticed that the sun makes you feel tired. And while we’d never choose bad weather, it’s not uncommon in the middle of a heatwave to wish for a break from soaring temperatures.

Aside from the fact we’re more active when the weather’s good, there are scientific reasons for why the sun makes us tired. Here, we explore those reasons, alongside tips on how to prevent sun fatigue and re-energise after a long day outdoors. 

Reasons the sun makes us feel tired

Before we can tackle how to stay energised after a day in the sun, we first need to know exactly why it makes us tired. Most often, it’s a case of biology; our bodies reacting to an environment we’re not used to. And if we’re to believe the stereotypes about UK weather, then we definitely are NOT used to sunshine on a regular basis. 

At its most basic level, tiredness is our body’s way of telling us it’s worked too hard. But what exactly is it about the sun that makes our bodies work harder than on a cool day?

1. Dehydration

When spending a day in the sun, our body needs more fluid than normal. If it doesn’t get the right amount, a day in the sun can result in dehydration. This simply means that we have lost more liquid than we have taken in. 

How serious this becomes really depends on the levels of liquid that we are losing and not replacing. Clearly, the longer we allow our body to lose more fluid than it takes in, the more serious our case of dehydration will be. While this may sound like something you’re already aware of, it’s surprising how often people get this wrong.

“Half of the people who come to me complaining of fatigue are actually dehydrated”, Dr. Woodson Merrell of New York’s Beth Israel Medical Center

dehydration symptoms to show what to look out for when feeling tired from the sun

Why does dehydration make us tired?

To understand why dehydration makes us tired, we first need to understand what’s in our blood! It’s common knowledge that blood is pumped around our body, sending oxygen to muscles and keeping us in fighting health. What’s perhaps surprising is the link between our water intake and how it affects our blood. 

When discussing blood, scientists split it into two distinct sections: solids and plasma. The solids part is made up of red blood cells, white blood cells and platelets. Most interesting is plasma, which accounts for over half our blood. This is the liquid part and contains 91-92% water. If our water intake drops, so does our volume of plasma.

With a drop in blood volume, our heart has to pump harder and faster to keep the body going. This extra exertion is why dehydration makes us tired. And to add insult to injury, a poor water volume in your blood can also cause a build-up of plaque in the arteries, making your heart work even harder! 

So, quite simply, water is a crucial component of our bodily function. When a day in the sun means you lose more fluid than you take in, you’re likely to feel more tired than usual. But this begs another question: why does the sun make our fluid levels drop? We’ll answer just that below.

2. Bodily temperature control

It’s no surprise that our body gets hot when it’s been in the sun. When this happens, our temperature control system kicks into action. This is controlled by a part of the brain called the hypothalamus which tells our sweat glands to get to work. This causes us to sweat. 

The sweat leaves your skin through tiny holes called pores. When the sweat hits the air, the air makes it evaporate (this means it turns from a liquid to a vapour). As the sweat evaporates off your skin, you cool down.” – kidshealth.org

The problem, as identified by How Stuff Works health journal, is when the air around us is humid, sweat finds it difficult to evaporate from our skin. This is because the air already has a high water percentage.

As with any process in the body, the harder we work in response to our environment, the more energy we use. And in the case of sweating, it’s not only that we use more energy. At the same time, we’re losing fluids which are crucial to our body working efficiently. And don’t just take our word for it, here’s what Dr.Waqas Ahmad, Family Physician, GP and Occupational Medicine Specialist had to say:

Cooling is actually a complicated process because, unlike heating, it doesn’t happen naturally. Heating occurs almost anytime there is a biological inefficiency but also occurs as a result of thermogenesis in humans. This heat production in the body excessively also makes a person tired and sleepy.

Cooling requires expending additional energyand resources in the form of sweat which is the first reason why you feel moreexhausted and sleepy. The act of sweating requires additional energy that your body wouldn’t be expending.

For completeness, sweating cools you down because of the latent heat of vaporization of water. When the water is evaporating from your skin, it draws the energy required to undergo the phase change from liquid to gas from your body. As it so happens, water has a very high latent heat of vaporization which makes this process pretty effective.

Is sweat the only part of the body’s temperature control system?

The body can lose heat through radiation too. This is where heat is transferred from our skin to another surface. Have you noticed how when you sit on a leather or metal chair, it takes a while for it to get warm? Or when you jump into bed on a cold night, it takes a while to warm up? 

This is because of radiation, which helps our bodies lose heat by transferring it to nearby surfaces. At all times our body radiates heat but when we are a higher temperature, it’s more extreme. Again, this uses up our energy reserves. 

3. Circadian rhythm and melatonin

The circadian rhythm is our internal clock. It’s the reason we wake up in the morning and go to sleep at night. It’s simply a sleep-wake cycle that responds to the environment around us. When it’s dark, it tells our body it’s time to sleep. And when the sun rises, our circadian rhythm tells us to wake up. It’s arguably the first type of alarm clock! Scientists believe the circadian rhythm was formed as a way to help our caveman ancestors regulate their sleeping patterns. 

circadian rhythm to show why the sun makes you tired

Not only does our circadian rhythm wake us up but it also impacts how alert or tired we feel. When light hits the retina in our eyes, signals are sent around the body that it’s time to work. This kicks into action a great many processes, from hormone production to metabolism and more. 

Related: How To Reset Your Body Clock

So how does this relate to why the sun makes us tired?

Well, not only does the body expect to work harder when in the sun but it also expects to work less at night-time. Without sunlight, our circadian rhythm puts our body into night-mode. This plays a role in why we feel tired when our day in the sun is over! 

4. Chemical changes

Speaking with the health team at Ezcareclinic, it became apparent that chemical changes caused by sunlight and heat also cause us to feel fatigued. Here’s what they had to say about why the sun makes us tired:

“It’s no secret that the sun’s ultraviolet rays can penetrate the skin and cause damage (such as sunburn, pigmentation changes, and wrinkles). The cascade of chemical changes that produce these effects can also cause drowsiness after hours in the sun.”

So, in brief, why does the sun make us tired?

When we are in the sun, our bodies work harder than they do in cooler weather. This is because our circadian rhythm kicks bodily functions into action, higher temperatures make us sweat to cool down, and we use more water which can cause dehydration. All this can cause chemical changes in our body which zaps us of our energy. Clearly, it’s really important to try and give our body a helping hand when we spend a day in the sun, but how do you do that? 

How to prevent feeling tired after a day in the sun 

There are some really simple methods to prevent feeling tired after a day in the sun. Some of these are obvious, while others are a little more surprising. Here, we explore 10 ways to prevent fatigue after a day in the sunny outdoors: 

  1. Drink plenty of water to replace lost fluids
  2. Apply sunscreen to lessen the impact of the sun on your body
  3. Wear loose clothing to keep your body open to the air – this helps maximise the efficiency of your sweat.
  4. Avoid caffeine as this makes your heart work harder when it’s already working hard!
  5. Avoid alcohol as it increases your chances of dehydration 
  6. Take frequent breaks from the sun and sit in the shade
  7. If there’s a swimming pool nearby, jump in it!
  8. If you don’t have a pool to jump in, take a cold shower
  9. Apply an ice pack to pulse points such as your neck and wrists. 
  10. Work in the mornings before the sun reaches its maximum strength.

How to re-energise after being in the sun

people in the sun

While there are many ways to prevent feeling tired after a day in the sun, you may still feel a little fatigued despite taking all the precautions. If so, all hope is not lost. One of the key parts of recovering from a hot day is replacing all the nutrients and water you’ve lost. Over at dailymeal.co.uk, Rachel Pack identifies the kind of diet you should consider after a sunny day: 

“First and foremost, look for food high in water content to stave off dehydration. Foods high in potassium, electrolytes, and, in many cases, a little sugar and simple carbohydrates will also help jump start your system back to life.”

Aside from your diet, there are other ways to manage tiredness after being in the sun. Taking a cold shower can help you re-energise, as can using hydrating face masks and skincare routines. You should also look to keep your mind active. Instead of sitting down in front of the TV, consider playing cards or a board game.

Do you have any tips or tricks to prevent feeling tired after a day in the sun? Let us know in the comments! 

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