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What should a baby wear under the sleep sack

Sleepsacks and swaddles are wonderful for babies and many times become a crutch for parents who love the additional sleep they get thanks to them. So, how do you know what your baby should be wearing under the swaddle?

The rule of thumb for newborn babies is that they should wear one additional layer in comparison to you and me. For example, if you’re comfortable wearing a t-shirt, your baby should be wearing a long sleeve onesie or two thin layers. This rule will vary a lot from baby to baby but it gives you an idea of where to start.

So how does this information help with knowing how to dress your baby under the sleep sack?

Knowing your baby and realizing that the sleepsack adds one layer of clothing is a good start. Since babies shouldn’t be sleeping with loose blankets until they are at least one year old, the sleepsack acts like the blanket layer. A onesie may be sufficient if it’s warm enough or you may choose to dress your baby in a footie pajama. If it’s a bit chilly you might consider layering a couple of items under the swaddle.

There are however a few other factors to consider when dressing your baby for sleep with a swaddle or sleep sack. The most important ones are:

  • Room temperature and humidity
  • The season and outdoor temperature
  • The material and thickness of the swaddle
  • How your baby looks and feels

We will take a look at them in more detail below, but first, let’s take a quick look at sleep safety with swaddles and sleep sacks.

Safe sleep in a swaddle or sleep sack

Per the American Academy of Pediatrics (AAP) guidelines, blankets or any other loose items in the crib are not safe for infants under 12 months of age. A swaddling blanket that may come loose in the night is just as dangerous as placing ablanket over your baby in the crib and may cause suffocation. 

This is where proper fitting swaddles and sleep sacks come in handy.

swaddled baby sleeping

A well-fitted swaddle with a strong velcro closing will not only keep your baby safe and warm at night but will also help curb the Moro reflex which is the number one cause of infant wake-ups during the first few months of life.

Swaddles and sleep sacks are not only safe, but they are also recommended by pediatricians as long as all other sleep safety guidelines are being followed.

Considerations for dressing your baby under a swaddle

Here are a few points to consider when deciding how to dress your baby for sleep in a swaddle or sleepsack.

Room temperature and humidity

The ideal room temperature for a baby to sleep as suggested by physicians is 68 to 70℉ (or 20 to 22.2℃). 

It’s also important to consider the level of humidity in the room. The heat combined with high relative humidity can make the room feel even hotter. The EPA recommends a relative humidity between 30% and 60%. 

Baby experts do suggest keeping the humidity closer to the upper end of the suggested range especially during winter time or colder periods.

If your baby’s room is temperature-controlled and you are able to keep it fairly stable, there is no need to overthink dressing your baby. A long sleeve onesie or footie pajama under a sleep sack should be more than sufficient. You can adjust the layers according to our guide below.

Season or temperature outside

Change in season can drastically affect the temperature outside and inside your home. If your home is not heat and AC controlled, you may need to check the weather forecastbefore deciding how to dress your baby.

Seasonality will also influence the relative humidity in the air. 

I like to keep a thermometer/hygrometer in the baby room where I can see from my baby monitor camera. There are also smart thermometer options that will let you monitor the temperature and set alerts if it goes above or below a set threshold.

Material and thickness of the swaddle

The material and thickness of the swaddle will also come into play. Is this a summer swaddle or a thick winter sleep sack? Does it match the season you are in and is it appropriate for the current room temperature

TOG – Thermal Overall Grade

TOG is a unit of measure used in the textile industry to calculate the thermal insulation of materials. Many baby swaddle manufacturers will use this measure to indicate the thickness and insulation abilities of their swaddle. 

Swaddle manufacturers such as Love To Dream make their swaddles and sleep sacks in different TOGs so they can be used at different temperatures.

A general guideline for TOG goes as follows:

  • 0.2 TOG – room temperatures between 75°F and 81°F
  • 1.0 TOG – room temperatures between 68°F to 75°F
  • 2.5 TOG – room temperatures between 61°F to 68°F
  • 3.5 TOG – room temperatures below 61°F

How does your baby feel to the touch?

This one may seem obvious but it is often forgotten by parents. Take a look at your baby, how does he look and feel? 

Check for the cheeks being flushed and the neck starting to feel sweaty as indications that your baby may be too hot. 

Observe and do not be afraid to check in on your baby to ensure he is comfortable throughout the night.

How to dress your baby under the swaddle or sleepsack

Here is a quick guideline to help you get an idea of how to layer your baby’s clothing according to the temperature in the room. Keep in mind this is only a guide and every baby is different. The way you decide to clothe your baby may vary according to their health and physiology in addition to the suggested temperatures below.

I’ve tried my fair share of swaddles. Some are better definitely better than others. Here is a short list of my favorite swaddles and why I love them:

  • Nested Bean Swaddle: I love the Nested Bean because it’s it comes in different TOGs and sizes and the material is really nice. The “bean” is also pretty nifty.
  • Zippy Swaddle: The Zipadee-zip is perfect if your baby is super strong and escapes most swaddles. It has a strip of material to help keep the arms down.
  • Nested Bean Zen Sack: The Zen sack is what I used from 6 to 14 months. It’s durable and super easy to use.
  • Zipadee-Zip transition sack: This is the perfect sleep sack for transitioning from a full swaddle to arms out. The material is so soft and stretchy.

Will a sleepsuit help keep my baby warm at night?

If you’re still worried your baby may be cold at night but still want to keep him swaddled, you may want to try a sleepsuit.

Baby Merlin’s Magic Sleepsuit works kind of like a swaddle but it gives your baby a little bit of additional freedom. It’s also a great transition away from the swaddle. Merlins magic suit comes in different materials and is perfect for babies that want to be fully covered.

Is baby warm enough in just a sleep sack?

That will depend on your baby’s temperament and physiology as well as the temperature and humidity in the room. Most babies will be comfortable in just a short sleeve onesie and a sleepsack at temperatures above 75℉ (22℃).

Signs a baby is cold in the crib

If you’re worried that your baby may be too cold I always suggest starting by checking the temperature in the room. Walk in the room or check the thermostat. 

After that you can verify visually if your baby is showing signs of distress. Here are some common signs that your baby may be feeling too cold.

  • Cold nose
  • Cold hands and feet
  • Pale skin
  • Fussy
  • Sudden Sneezing

Signs a baby is too hot in the crib

It is also possible that your baby is overdressed for the weather and is feeling too hot. Overheating can increase the risk of SIDS so it is important to keep an eye out for the following signs that your baby is too hot. 

  • Flushed skin
  • Rapid heartbeat
  • Restless
  • Sweaty nape of the neck

Stay Cool

When it comes to baby sleep it’s always better (and safer) to stay on the cooler side if you have a way to control the temperature in the room. Newborn babies don’t have great body temperature control so make sure to keep an eye on your baby and dress him appropriately for the weather, temperature and humidity in your home.