Why Does the Sun Make Me Tired?

Whether it’s a vacation, a 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 that we’re more active when the weather’s good, there are scientific reasons 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.

One of the biggest reasons you feel sleepy after being in the sun is that your body is working hard to maintain a normal internal temperature. This process is called thermoregulation. When you get hot, your brain’s hypothalamus signals your body to cool down. This triggers two main responses:

• Sweating: This is your body’s primary cooling mechanism. As sweat evaporates from your skin, it absorbs heat. But this process isn’t passive; it requires energy, and your metabolic rate actually increases.
• Vasodilation: Your blood vessels widen to bring more blood to the surface of your skin, which helps release heat. This is why you might look flushed. While effective, this process lowers your blood pressure and forces your heart to pump harder and faster to circulate blood, which uses a lot of energy.

When it’s humid, this process is even less efficient. The moisture already in the air slows down the evaporation of your sweat, forcing your body to work even harder to cool off, zapping even more energy.

When spending a day in the sun, our body loses a lot of fluid through sweat. If you don’t replace that fluid, you quickly become dehydrated. But why does dehydration make us tired? Dehydration causes a drop in blood volume, which means less blood flows to your brain and other organs. Your heart then has to work harder to pump the remaining blood around your body, leading to feelings of fatigue. In fact, many people who complain of tiredness are often found to be simply dehydrated.

It’s not just water you’re losing, either. When you sweat, you also lose essential minerals called electrolytes, like sodium and potassium. These are crucial for muscle function and energy. Losing them contributes significantly to that feeling of sluggishness.

Even a mild sunburn can make you feel tired. When your skin is damaged by UV rays, your body initiates an immune response to heal it. This healing process requires energy. Your body diverts fluids to the sunburnt area, which can further contribute to dehydration. A sunburn also raises your overall body temperature, forcing your cooling mechanisms to work even harder.

Our internal body clock, known as the circadian rhythm, helps regulate sleep, alertness, hormone levels, and even digestion. One of the key players in this system is melatonin, the hormone that signals to your body when it’s time to sleep.

When you’re exposed to bright sunlight, especially in the morning and early afternoon, your body suppresses melatonin production and increases levels of serotonin, the hormone linked to mood, energy, and focus. This is why you often feel alert and upbeat in the sunshine.

But later in the day, the relationship between these hormones can shift. As daylight fades, your body begins converting serotonin into melatonin in preparation for sleep. This can sometimes lead to a drop in energy or mood, especially if you’ve had prolonged sun exposure that shifts your internal rhythm slightly. Additionally, being outdoors late into the evening, when natural light levels are dropping, may delay melatonin release, making it harder to fall asleep at your usual time.

So while the sun initially boosts your energy, it can also leave you feeling unexpectedly drained as your circadian rhythm adjusts.

It’s crucial to know the difference between feeling a bit tired from the sun and suffering from a serious heat-related illness like heat exhaustion. While sun fatigue is normal, heat exhaustion happens when your body can no longer regulate its own temperature. According to the NHS, signs of heat exhaustion include:

• Headache
• Dizziness and confusion
• Loss of appetite and feeling sick
• Excessive sweating and pale, clammy skin
• Cramps in the arms, legs, and stomach
• Fast breathing or pulse
• A high temperature of 38°C or above
• Being very thirsty

If you or someone else has these signs, always seek professional medical advice.

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 that zap us of our energy. It’s really important to try to give our bodies a helping hand when we spend a day in the sun, but how do you do that?

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 ways to prevent fatigue after a day in the sunny outdoors:

• Stay hydrated: Drink plenty of water before, during, and after sun exposure. Don’t wait until you feel thirsty.
• Replenish electrolytes: Since you lose electrolytes through sweat, consider a sports drink, coconut water, or eating foods rich in potassium like bananas and watermelon.
• Apply suncream religiously: This minimises sunburn, which saves your body the energy it would need for healing.
• Wear loose, light-coloured clothing: This helps sweat evaporate efficiently and reflects sunlight.
• Avoid alcohol and caffeine: Both are diuretics, meaning they make you lose more fluid, worsening dehydration.
• Seek shade frequently: Take regular breaks out of direct sun, especially between 11 am and 3 pm when the sun is strongest.
• Take a cool shower or swim: This helps lower your body temperature quickly, giving your internal systems a break.
• Time your activities: If you have strenuous work to do, do it in the cooler morning or evening hours.
• Eat lighter meals: Heavy foods require more energy to digest. Opt for fresh salads, fruits, and lean proteins like fish.

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.

To re-energise, focus on foods high in water content to combat dehydration. Foods high in potassium and other electrolytes 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. 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.

Finally, prioritise a good night’s sleep to allow your body to fully recover and repair.

Need some help sleeping during warm summers? We’ve got 30 tips to cool down at night to make the best of that sunshine sleepiness.