Why Does the Sun Make You Tired?

Exposure to the sun can leave you feeling unexpectedly tired, thanks to several things happening at once. Your body works hard to stay cool, and it loses fluid through sweating. On top of that, your hormones and sleep shift with the light and the heat. Together, they can really drain your energy.

Whether it’s a holiday, a staycation or a warm summer’s day, you’ll have noticed the sun leaves you feeling sleepy. And while we’d never wish for bad weather, it’s not unusual in the middle of a heatwave to long for a break from the heat. Beyond simply being more active when the sun’s out, there are real reasons behind that drowsiness. Here they are, along with tips on preventing sun fatigue and picking yourself back up 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 hold a steady internal temperature. This process is called thermoregulation. When you heat up, your brain’s hypothalamus signals your body to cool down, triggering two main responses:

• Sweating: This is your body’s main way of cooling down. As sweat evaporates from your skin, it carries heat away, but it isn’t a passive process. It takes energy, and your metabolic rate actually rises.
• Widening blood vessels: Your blood vessels open up, a process called vasodilation, to bring more blood to the surface of your skin and let heat escape. It’s why you go flushed. It also lowers your blood pressure and pushes your heart to pump harder and faster, which burns a lot of energy.

Humidity makes it harder still. The moisture already in the air slows the rate at which your sweat evaporates, so your body has to work even harder to cool off, draining yet more energy.

Spend a day in the sun, and your body loses a lot of fluid through sweat. If you don’t replace it, you quickly become dehydrated. So why does that make you tired? Dehydration causes a drop in blood volume, which means less blood reaches your brain and other organs. Your heart then has to work harder to move what’s left around your body, and that leaves you feeling worn out. In fact, even mild dehydration is a common and easily overlooked cause of feeling tired.

It isn’t only water you lose, either. When you sweat, you also lose essential minerals called electrolytes, like sodium and potassium. These are important for muscle function and energy, so losing them adds to that sluggish feeling.

Even a mild sunburn can leave you tired. When UV rays damage your skin, your body kicks off an immune response to heal it, and that healing takes energy. It also diverts fluid to the burnt area, which can add to dehydration, and it raises your overall body temperature, so your cooling system has to work harder again.

Your internal body clock, known as your circadian rhythm, helps manage sleep, alertness, hormones and even digestion. One of its key players is melatonin, the hormone that tells your body when it’s time to sleep.

When you’re out in bright sunlight, especially in the morning and early afternoon, your body holds back melatonin and produces more serotonin, the hormone tied to mood, energy and focus. That’s why you often feel bright and upbeat in the sunshine. Later on, as the light fades, your body starts turning serotonin into melatonin to get ready for sleep, which can bring a dip in energy or mood, especially after a long stretch of sun. Staying outdoors late, as the light drops, can also delay melatonin and nudge your rhythm out of its usual pattern.

It isn’t just the daytime that catches up with you, either. Hot, sticky nights make it harder to sleep well, because a warm room increases wakefulness and eats into your deep and REM sleep. Our 2026 Sleep Survey found that being too hot is one of the top reasons people give for a broken night, named by nearly 1 in 4 of us (24%). So a heatwave can leave you tired the next day before you’ve even stepped outside. Getting your bedroom temperature right and switching to cooling bedding makes a real difference.

It’s important to know the difference between feeling a bit tired from the sun and a serious heat-related illness like heat exhaustion. Sun fatigue is normal. Heat exhaustion is when your body can no longer keep its own temperature in check. According to the NHS, the signs of heat exhaustion include:

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

If you or someone else has these signs, cool down and rehydrate. Get medical help if there’s no improvement within 30 minutes.

Some of these are obvious, and some are a little more surprising, but they all help you stay brighter after time 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.

Even with all the precautions, you might still feel a little flat after a day in the sun. If so, all is not lost. A big part of recovering is replacing the water and nutrients you’ve lost, so lean on foods with a high water content and plenty of potassium and other electrolytes to get your system going again.

Beyond your diet, a cool shower can perk you up, and keeping your mind gently busy helps too. Rather than slumping in front of the telly, try a few hands of cards or a board game. Most of all, give yourself a proper night’s sleep so your body can fully recover, ideally in a cool, dark room. If the heat is making that tricky, our 30 tips to cool down at night and our guide to sleeping well in any climate can help you make the most of that sunshine sleepiness.