Why Does the Sun Make You Sleepy?

Lucy Bell-Young

by Lucy Bell-Young

4th July 2018

As ironic as it sounds, lying in the sun all day can be exhausting for your body. The Spanish cracked this problem with their afternoon siestas, but why does a sunny day make us so sleepy?

That must-nap-now feeling that takes over you on a hot day is caused by a number of factors. In the heat, your body is working even harder than usual to maintain a consistent temperature. This causes a range of physiological changes and these can actually be very tiring.

  1. Body Temperature

Your body is always working hard to maintain a consistent body temperature of around 37°C (98.6°F). But when you spend your time outdoors on a hot and sunny day, your body has to work much harder to regulate your internal temperature. This is because of exposure to direct heat.

When you’re in direct heat, radiation from the sun can quickly increase your internal temperature. This means that your body is working overtime to cool it back down to the normal range. It does this by the radiation of heat through the skin, as well as the evaporation of sweat.

It takes a lot of physiological effort for your body to cool you down when it’s very hot. A lot of energy is used up and your heart rate as well as metabolic rate increase significantly. This is what makes you feel sleepy and exhausted, even if you’ve been doing absolutely nothing.

Sky shot of a woman sun bathing under a palm tree

Even if you’re just lying down on a beach, your body is working very hard to maintain a constant internal temperature. This is what makes you sleepy.

  1. Dehydration

It’s a well-known fact that heat makes you sweat, even if you’re just tanning. The more you sweat, the more water, salts, electrolytes and other fluids you lose. This can cause dehydration if you’re not replacing the fluids that are getting lost.

You become sleepy when you’re dehydrated because your blood volume decreases. This means that there is not as much blood reaching your brain, causing your heart to pump faster and harder. Your body also uses up a lot of energy to maintain your temperature and this can also lead to dehydration. This is why it is important to keep drinking water on a hot day.

It is also important to replenish your electrolyte levels, mainly sodium (Na+) as this helps to regulate the fluid balance in your body. You can boost your sodium levels by snacking on something salty or having a few sips of a sports drink.

  1. Sunburn

A sunburn does more than just turn you bright red and cause your skin to peel. It also raises your body temperature and can dehydrate you very quickly.

In response to a sunburn, your body diverts fluid away from the rest of the body and towards the affected skin surface. This makes it very difficult for your body to keep you hydrated while maintaining a constant internal temperature. With these two factors combined, your body becomes exhausted and you become sleepy.

  1. Melatonin

Melatonin is the body’s sleep hormone that emerges when it’s dark. The production of melatonin tells your body that it is time to rest and this is what makes you sleepy. It is why we spend the long, dark months of winter in a slump, and also why we experience the spring fever boost.

Being out in the sun all day inhibits your melatonin production, especially since the days are longer and brighter during summer. Therefore, when night finally comes, the effect of melatonin is far more pronounced, making you feel suddenly sleepier than usual.

A beach with a bright sun over the sea

You become sleepy in summer because of how your body responds to increased temperature, dehydration, sunburn and even melatonin.

Heat Stroke and Exhaustion

If you’re feeling unusually sleepy after a long day spent in the sun, it’s important to keep an eye on any symptoms that could be related to heat exhaustion or heat stroke.

Heat exhaustion essentially happens when your body overheats. It is characterised by nausea, fatigue, weakness, dizziness and profuse sweating. It is your body’s response to an excessive loss of fluid and can result in collapsing or fainting.

Heat stroke is more severe. It is a form of hyperthermia where your internal temperature dramatically increases and your body becomes very dehydrated. In normal circumstances, your body is able to easily dissipate the heat that it generates through metabolism. It mainly does this by sweating.

However, when it is extremely hot and you’re exposed to direct heat, it becomes difficult for your body to sufficiently dissipate this extra heat. This results in the body temperature rising as it can’t be cooled down fast enough.

The body temperature can increase to 41°C (106°F) during heat stroke, and many classify this temperature range as severe hyperthermia.  Symptoms of heat stroke include:

  • Absence of sweat
  • Rapid pulse
  • Confusion
  • Disorientation
  • High body temperature
  • Seizure

Heat stroke has the potential to put you in a coma, and if left untreated it can be fatal. This is why it is very important to make sure you stay well hydrated and give your body a chance to cool down during hot spells.

A sleepy puppy resting on some cushions

You may have been waiting all year to finally achieve that golden-brown tan, and with your extra sleepiness, it may be tempting to take a quick kip on your lilo. But remember to always wear sunscreen, drink water and cool down every once in a while!


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