Why Does Heat Make You Sleepy?

why does heat make you sleepy

Have you ever felt lethargic and confused while walking outdoors during the day? Wondering why does heat make you sleepy?

It is common for people to feel tired, sleepy, lethargic, or confused during the summertime, mainly if their work involves staying outdoors for a long time. Just think about it. Even when you spend 1-2 hours outdoors, in the sun, the internal temperature of your body rises drastically as it is trying hard to regulate heat. 

Ordinarily, the internal body temperature of a human (also known as normothermia or euthermia) ranges somewhere between 36.5–37.5 °C (97.7–99.5 °F). As you step out of your home during the day, it rises by a few decimal points.

However, in certain conditions, the internal body temperature may increase to 41 °C (105.8 °F). This is where things begin to go south. This condition is known as Hyperthermia (or heat stroke). 

The more time you spend out in the sun, the more you feel sleepy and tired. This is due to the depletion of the oxygen levels in your cells. At this stage, most people start sweating profusely or take a break from whatever they are doing. But things can go wrong if they continue to let the internal body temperature rise further. In some cases, people even die from a heat stroke.

Why does heat make you sleepy?

Several pseudoscientific theories arise when you ask the question, “why does heat make you sleepy?” While researchers believe heat to be detrimental for good quality sleep, to understand the real reason, you might need to learn a bit about an enzyme known as “Pyruvate Kinase.” Pyruvate kinase is responsible for glycolysis. 

Let’s take a closer look at why does heat make you sleepy? The heat radiating from the sun hinders your body’s ability to produce ATP (Adenosine triphosphate). ATP is the energy molecule present in the cell. When we eat, the food gets converted into glucose and further gets consumed by your cell via ATP. 

However, due to heat, pyruvate kinase (which is an active agent in the production of glucose) fails to do its job after about 41 °C (105.8 °F). This slows down the metabolism of your body, and it enters into an energy conservation state. Hence, you feel sleepy and tired. Now, whether heat makes you sleep well or not is another question.   

It should be noted that when you spend too much time in the sun, it is natural to feel a little jaded and tired for the rest of the day. Tiredness tempts people to want to take a nap or just lay in bed for a few minutes. What I’m leading up to is this: Just because heat makes you feel sleepy does not mean it promotes good quality sleep. So, a quick nap should be enough to feel fresh and vibrant again. 

Here are a few common reasons that can hinder the process of “glycolysis” and make you feel sleepy

1) Dehydration

Dehydration occurs due to the depletion of water from your body. During summers, particularly in the warm parts of the world, it is common for people to lose liters of water via sweat. 

Headache is the most common symptom of dehydration. Some people also suffer from muscle pangs. So, if along with drowsiness, if you show the two signs mentioned above, then you must immediately rehydrate yourself with a glass of cold water. 

Headaches caused by dehydration should immediately stop after rehydration. If the symptoms persist, it is better to consult a physician as the cause may be something other than dehydration.

2) Intense physical activity

Why does heat make you feel sleepy

Do you workout in the gym? Have you recently made any changes to your workout schedule?

Most people workout after work hours as it allows them to relax and chill for a few hours before going to bed. There is nothing wrong with working out in the dusk. However, heat can dictate how you feel if you prefer to go to the gym in the morning or afternoon.

If you are facing this issue, we recommend engaging in cardiovascular exercises like running or spinning. Doing more cardio will increase the flow of oxygen through your body and prevent after workout burnouts. You will feel more energetic and less sleepy than you do at the moment.

3) Staying indoors during winters

We all know how cozy and comfy it feels to stay in your home during the winters. Most people spend the entire season either relaxing at home or working in offices with the heating system running on full throttle. This is the only occasion when heat feels soothing. Staying all day indoors makes you feel weak, fatigued, and sluggish. 

4) Melatonin

Melatonin is a hormone released into the body by the pineal gland–located in the brain. This hormone regulates the sleep-wake cycle or the biological clock. Plus, sleeping pills contain synthetically made melatonin. 

When the sun goes down, the pineal gland becomes active and begins secreting melatonin in the body–via blood. Melatonin levels in the blood are known to be highest around 9-10 PM. However, the levels drop drastically during the daytime — especially when you are out in the sun.

The sudden exhaustion of melatonin creates an imbalance, and the pineal gland releases a high dosage the moment you return home. Hence, you feel sleepy on coming back from a mid-day outing.

I intend to stay alert during the day and not feel sleepy. How to keep fresh and active despite the warm weather? 

1) Drink plenty of water

During summers, it is essential to stay hydrated by taking frequent water breaks. Whether you work outdoors or indoors, the high temperature can sap away all your energy within minutes. Observing the color of your urine is the best way to check if you are drinking enough water or not. 

Here is a Urine Color Chart by NSW to help you out. Ideally, your pee should always be colorless. 

2) Spend more time in an air-conditioned place

The more you stay outdoors, the more body fluids you lose. During summers, it is essential to spend as much time in an air-conditioned place like a mall, cafe, library, etc.

Some of these places also allow you to charge your laptop battery. So, why not make the best of the opportunity by cooling off in between your work sessions. 

Heat can also be a problem while you are indoors (in your home, office, etc.). So, make sure the air conditioner is set to cool mode. Moreover, cover the windows with curtains to prevent direct sunlight from reaching your home. The lower the temperature of your house, the better you will feel. 

3) Take regular evening walks

Just because you can’t go out in the day does not mean you should become a “homebody.” Head out a few minutes after the sun has gone down. Take solo walks with your dog or go out with friends. You can also ride a bike if that suits your lifestyle.

Further, on weekends, you can take your entire family to a nearby public pool. Public pools are great for cooling off, also meeting other people in your community. Swimming at least once a week will bring up your heart rate, which in turn will make you active.

Swimming also helps you shed those unsightly extra pounds that you are likely to gain during the rest of the week. Hence, when you have so much to do after sundown, you no more have to go out during the day. With time, you will stop wondering “why does heat make you sleepy?”


I hope the above post raised your awareness about sleep health during summers. To reiterate, feeling sleepy due to heat is mainly due to an imbalance in the melatonin levels in your blood.

You don’t have to obey the urge to sleep by actually going to bed during the day. Have some cold-brew, spend time in the shade, and you will be good to go.