🌞 Introduction
In summer, it's important not only to decide whether to use air conditioning but also to adjust your clothing, bedding, and hydration.
Babies and young children have less developed thermoregulation than adults.
Even when using air conditioning to avoid the heat, body heat can build up from wearing too many layers of clothing or using too many blankets.
This page summarizes summer clothing, bedding for sleeping, sleepwear, hydration, and the common question, "Will I catch a cold if I sleep with the window open?"

👕 Clothing: "About one layer more than adults"

A good rule of thumb is to dress a baby in one more layer than an adult would wear to be comfortable.
During hot weather, it's important not to overdress them.
If you think, "I might be cold because the air conditioning is on," and you wear too many layers, your body can overheat.
Choose lightweight, breathable clothing.
There's no need to keep wearing a hat indoors.
Wearing a hat indoors continuously can cause heat to build up.
The guideline for clothing is to wear one more layer than an adult.
On hot days, choose thin, breathable clothing.
Even when using the air conditioner, be careful not to overdress.
Avoid keeping hats on indoors for extended periods.

Let's check for signs of overheating.

You can't tell if a baby is too hot just by their clothing.
Let's check for sweating, redness of the face, heat in the chest, etc.
The literature also lists sweating, a flushed face, and a chest that feels hot to the touch as signs of overheating.
Just because your hands and feet feel a little cold doesn't necessarily mean you're cold.
When checking on your baby, it's good to look not only at their arms and legs, but also their chest and back, for sweat, complexion, and general liveliness.
I am sweating.
Your face is red.
My chest is hot to the touch.
Seems to be having trouble sleeping.
I'm exhausted.
If it looks like this, it might be too hot.
Let's adjust by wearing lighter clothing, lowering the room temperature, and using a dehumidifier.

When you sleep, be careful not to overdo it.

You need to be careful not to overheat while you're sleeping.
Especially in infants, thick bedding, blankets, pillows, and stuffed animals can cover the face and lead to suffocation or accidents during sleep.
During hot weather, adjust your temperature with thin clothing rather than thick blankets.
For the bedroom, aim for an actual room temperature of around 22-24℃ and humidity of about 40-60%.
This is not the air conditioner's set temperature, but a guideline for the actual room temperature to check with a thermometer.
Adjust while checking if the baby is sweating, seems to be having trouble sleeping, or if their chest and back are hot.
🔸Try not to use too many thick comforters or blankets.
Avoid covering your face with bedding.
🔸Avoid placing pillows or stuffed animals in the crib.
When it's hot, adjust by wearing lightweight clothing.

Choose the right thickness for your sleeper based on the season.

Sleepers have the advantage of not easily covering the face like a regular blanket.
However, this does not mean using a thick sleeper during hot weather.
When using it in the summer, choose something thin and breathable.
Using a thick winter sleeper in the summer can cause heat to build up.
Even when using a sleeper, check for sweat, facial redness, and warmth on the chest and back; if your child seems hot, either avoid using it or switch to a lighter-weight one.
🔸Sleepers have the advantage of not easily falling onto your face.
In summer, choose lightweight and breathable materials.
Thick sleepwear can trap heat.
If you feel hot, reduce your clothing and bedding.
For your usual fluid intake, water and tea are the basics.

During hot weather, frequent hydration is important.

During hot weather, frequent hydration is important.
For children under 6 years old, water or tea is generally sufficient for activities like playing outside or tag.
Sports drinks are beverages used during prolonged, intense exercise or when sweating heavily.
Drinking sports drinks and juices daily can lead to excessive sugar intake.
Let's drink before playing outside.
🔸Let's call out to them while they're playing.
Let's drink after returning home.
For children who can't drink a lot at once, divide a small amount into several portions.
On hot days, don't just wait for them to say they're thirsty; adults should also remind children to drink frequently.

Baby Hydration
For babies who are drinking breast milk or formula, their feedings and milk are generally sufficient for hydration.
If your baby is drinking enough breast milk or formula, they often don't need extra water.
If you've started solid foods, you can use a small amount of water or barley tea as needed.
However, the appropriate response will vary depending on the baby's age, weight, feeding amount, and whether they have a fever.
If your baby is feeding poorly, producing little urine, or seems listless, consult a pediatrician promptly.

When you only drink juice or sports drinks
Some children dislike water and tea and only drink juice and sports drinks.
In that case, it's important to gradually adjust the home environment rather than banning it all at once.
Here are some strategies: reduce the amount of sweet drinks kept at home, place water and tea in visible locations, have guardians drink water and tea with children, and use favorite water bottles or cups.
Children's drink preferences are also influenced by their home environment.
Let's not have only the children push themselves, but let's have the whole family make gradual adjustments together.

Is it true that "I caught a cold because I slept with the window open"?

"I caught a cold because I slept with the window open." I sometimes hear people say that.
However, a direct causal link between sleeping with the window open and catching a cold has not been clearly proven medically.
A cold is basically an upper respiratory tract infection caused by a virus.
Cold air itself does not directly cause colds.
On the other hand, in environments that are too cold, or in low-temperature, dry environments, the protective functions of the nose and throat can be weakened, potentially making you more susceptible to respiratory infections.
Therefore, rather than thinking "I'll catch a cold because I opened the window," it's important to create an environment that isn't too cold.
During hot weather, make sure to use ventilation and air conditioning effectively to create a comfortable indoor environment for your child to sleep in.

Signs to consider seeing a doctor

When you observe the following, there is a possibility of heatstroke or dehydration.
I'm exhausted.
Poor response.
You look pale.
I can't drink water.
Urine has not been passed for more than half a day.
Even when I cry, I don't shed many tears.
My mouth is dry.
Vomits repeatedly.
I have a high fever.
Infant with markedly poor feeding.
When you are worried, contact a pediatrician or the emergency consultation service as soon as possible.

Summary
For babies and infants in summer, it's important to adjust not only the air conditioning but also their clothing, bedding, and hydration.
As a guideline, dress your child in one more layer than you would wear yourself. On hot days, choose thin, breathable clothing.
When you sleep, avoid using too many thick futons or blankets, and avoid covering your face with bedding.
For your usual hydration, stick to water and tea, and limit your intake of juice and sports drinks.
Sleeping with the window open doesn't directly cause a cold, but it's important to create an environment that isn't too cold.
When you're unsure, adjust by checking your baby's chest and back for warmth, sweat, flushed cheeks, and overall energy.

References
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Darrow HJ, Carman KA, Wheeler V. Sudden Infant Death Syndrome: Common Questions and Answers. American Family Physician. 2025.
Rowland T. Fluid Replacement Requirements for Child Athletes. Sports Medicine. 2011.
Eccles R, Wilkinson JE. Exposure to Cold and Acute Upper Respiratory Tract Infection. Rhinology. 2015.
Mäkinen TM, Juvonen R, Jokelainen J, et al. Cold Temperature and Low Humidity Are Associated With Increased Occurrence of Respiratory Tract Infections. Respiratory Medicine. 2009.
Wolkoff P, Azuma K, Carrer P. Health, Work Performance, and Risk of Infection in Office-Like Environments: The Role of Indoor Temperature, Air Humidity, and Ventilation. International Journal of Hygiene and Environmental Health. 2021.
Arundel AV, Sterling EM, Biggin JH, Sterling TD. Indirect Health Effects of Relative Humidity in Indoor Environments. Environmental Health Perspectives. 1986.