Why You May Feel More Tired in the Heat And How to Stay Energized

Key Takeaways

• Hot weather drains your energy as your body works harder to regulate its temperature.
• Dehydration and shifts in blood glucose levels can exacerbate summer fatigue.
• Smart habits, such as proper hydration, balanced snacks, planning outdoor activities, and being aware of warning signs, can help keep you energized and safe in the heat.

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Spending a summer day outside at the beach, lake, or working in your yard can zap your energy. You may wonder why this happens and what the impact is on your body. 

Sunlight is beneficial for many reasons. It helps to regulate cycles of alertness and sleep through the circadian rhythm. The circadian rhythm is the 24-hour internal clock in our brain that also regulates body functions.¹ But the sun also produces heat, which can cause summer fatigue. 

This article helps you understand how hot temperatures can cause fatigue and the multiple physiological pathways through which heat causes your body to use more energy and reduce alertness.

Thermoregulation Overload: Your Body's Heat ‘Engine’ Burns Energy

When the temperature outside rises, your body maintains its internal temperature in several ways.² 

Your body maintains a core body temperature range of 97.7 to 99.5°F. When it senses your temperature rising, your body responds by:

• Making you sweat more, which releases heat.
• Sending blood to the surface of the skin so heat can escape (increased heart rate).
• Slowing down your metabolism so you make less heat on your own.
• Stimulating behavior changes like moving less, stretching out to release heat, removing extra layers of clothing, and reducing appetite.² 

These pathways use energy to cool off your body and encourage you to move less in response. This results in more energy burned to regulate your temperature, which can lead to feeling tired, especially if you are not fueling and hydrating properly in the heat. 

Dehydration and Electrolyte Loss: Thirst Isn’t Enough

As your body sweats more to maintain a stable body temperature, you lose not only fluid but also minerals such as sodium, chloride, potassium, and, to a lesser extent, magnesium and calcium.

When exercising or working in the heat, you can lose 1 to 3 liters of fluid per hour. Each liter of sweat contains approximately:

• 460 to 1,840 mg of sodium
• 710 to 2,840 mg of chloride
• 160 to 390 mg of potassium
• 0 to 36 mg of magnesium
• 0  to 120 mg of calcium³  

Sodium, chloride, and potassium regulate and maintain proper fluid balance. Magnesium and calcium play crucial roles in muscle function and energy metabolism.³

When fluids and electrolytes are not replaced after sweating, varying degrees of dehydration can occur. For dehydration, sodium levels tend to be higher in the blood since fluid is lost and blood volume has decreased.⁴ 

A 2% weight loss (about 3 to 4 pounds lost for adults weighing 180-200 pounds) indicates dehydration or impending dehydration. Symptoms of mild dehydration include thirst, dry mouth, fatigue, and slight changes in blood pressure.⁴ 

You can usually resolve mild dehydration at home with oral rehydration solutions that contain water, electrolytes, and sugar.⁴

Once the severity increases, more symptoms occur, such as dizziness, low blood pressure, confusion, and severe lethargy (extreme tiredness), which will likely require medical intervention.⁴ 

Heat, Blood Sugar, and Metabolic Slumps

During heat exposure, dehydration causes fluid loss from the blood, resulting in more concentrated blood glucose levels, which can lead to elevated blood glucose levels and trigger fatigue in individuals with and without diabetes. Technically, there isn’t more glucose in the blood, but the ratio of glucose and fluid is off.⁵

High glucose levels reflect glucose (the body's primary energy source) remaining in the blood and not being shuttled to the cells and tissues that need it, including the brain, muscles, nerves, and red blood cells. Less energy for these cells makes you feel tired as well.⁶ 

Correcting mild dehydration can help blood glucose levels return to normal, allowing cells to utilize the energy in the blood.^5,6

Circadian & Brain Temperature Signals: Time for a Siesta?

Heat can trigger your brain and body to seek rest. As mentioned earlier, when the body senses a higher temperature, signals are sent to the body to rest, stretch out to cool off, and remove extra clothing.²

Research also shows that other animals reduce movement and sleep more when the temperature rises. A 2022 study asked 818 dog and cat owners to reflect on their pets’ behavior. It found that dogs and cats play and move around more when it’s cold, but they rest and sleep more when it’s very hot (or very cold).⁷ 

Other research on fruit flies found special brain cells that detect the temperature and turn on during heat and off during cooler weather. When these cells are activated, the fruit fly will increase its midday sleep (or siesta) to cope with the heat.⁸

Heat can encourage a nap during the hottest part of the day to help humans and animals cope with the intense temperatures.

Heat Illness Spectrum: When Fatigue Becomes Danger

Dehydration and heat illness can become a dangerous situation if you fail to heed your body's cues to rest and hydrate (or if you are unable to do so). 

When the body is unable to regulate its core temperature after being exposed to heat, it can result in heat exhaustion or heat stroke. 

Heat exhaustion occurs after excessive loss of water and sodium chloride (salt) through sweating. Symptoms include headache, nausea, dizziness, weakness, thirst, excessive sweating, and elevated body temperature. Treatment includes moving the person out of the heat, cooling the body with cold cloths, and frequent sips of liquids.⁹ 

Heat stroke is a more severe condition that may cause confusion, slurred speech, loss of consciousness, hot and dry skin, seizures, and extremely high body temperatures. It can be fatal if treatment is delayed. Emergency medical care is typically required to help the body return to normal function and regulate its core temperature.⁹ 

How to Stay Energized in Hot Weather: Smart Strategies

These preventive measures can help you stay energized in hot weather and prevent heat-related illnesses. 

• Hydrate early and often: Plan for hot weather by hydrating before being in the heat and then sip water and electrolytes while in the heat to prevent thirst (a sign of dehydration).
• Stay cool: limit sun by planning outdoor activities earlier in the day, wear lightweight clothes, use shade from trees or umbrellas, and keep the air moving with fans. 
• Support stable blood glucose levels: Eat balanced meals and snacks with protein, fiber, healthy fat; monitor glucose levels if at risk (people with pre-diabetes or diabetes).
• Adjust your outdoor physical activity: Opt for light movement (such as walking or swimming) rather than intense exercise in the heat. 
• Rest strategically: Take a short nap or schedule frequent indoor breaks to refresh, preventing drowsiness and dehydration.

Adjusting your physical activity routine when the temperature heats up can help you stay energized and feeling great.

When to Seek Help

If you do spend time outside in the heat, pay attention to red-flag symptoms:

• Extreme fatigue
• Heavy sweating
• Dizziness
• Confusion
• Slurred speech
• Fast heartbeat
• Nausea
• Weakness

These could indicate heat exhaustion, heat stroke, or low blood glucose levels. Slight tiredness from being in the heat is expected, but it can quickly turn into situations requiring medical care if you fail to rest, hydrate properly, and cool off regularly.^8,9 

The Bottom Line

Hot weather can drain your energy because your body works harder to stay cool, and your body loses fluid and electrolytes. Your body also adjusts its energy intake and usage when it's hot, and your brain signals you to rest more.

Understanding your body and the factors that drive heat-induced fatigue, thermoregulation, dehydration, metabolic shifts, and neurological triggers can help you make informed adjustments. Listen to your body and stay hydrated, eat balanced snacks, take breaks, and plan outdoor activities wisely. These smart habits can help you stay energized, safe, and ready to enjoy summer. 

Learn More With Signos’ Expert Advice

A CGM lets you see how your blood glucose responds in real-time to heat, dehydration, food, and physical activity. Since hot weather and dehydration can cause your glucose level to become more concentrated, a CGM can help you catch those shifts earlier. 

You can then adjust your eating, hydration, and time spent outdoors in the heat to make wise choices during hot weather and improve your health. Learn more about glucose levels and tracking on the Signos blog, written by health and nutrition experts.