What to Wear Under a Sleep Sack: A Practical Guide for New Parents

Happy baby wearing a Woolino 4 Season Ultimate Baby Sleep Bag layered over a long sleeve onesie.

4 Season® Ultimate Baby Sleep Bag, Merino Wool, 2 Months - 2 Years, Birch Gray - $109.00

A lot of parents feel stumped when they first attempt to dress their babies for bed - because what do you put them in? How do you make sure that they are going to be warm enough, cool enough, comfortable enough, and safe throughout the night?

Many new parents opt for sleep sacks as a safe, convenient, and practical option. These are a wonderful starting point because they provide a whole host of benefits, including increased safety and warmth. They help the baby stay at the right temperature throughout the night, but what you put underneath them is also very important, which leaves some people wondering, what should a baby wear under a sleep sack?

Woolino provides a variety of specialized baby clothing that is suitable for wearing under sleep sacks, all made using natural merino wool. With these options, it’s easy to get the right amount of layering to ensure your little one is comfy all night long!

The Importance of Correct Layering Under a Sleep Sack

As an adult, you have the ability to regulate your body temperature pretty effectively, through shivering if you’re cold, sweating if you’re hot - or simply getting up and altering the amount of clothing you wear. 

Babies are less effective when it comes to controlling their body temperatures, as it takes time to develop these responses, and because they have a much greater surface area compared to body weight than an adult. 

This means that the clothing they wear is even more important. It’s common to start dressing a  baby in a sleep sack from birth or after they’ve finished swaddling, so knowing the safest way to use them is crucial.

As a parent, the best thing you can do to keep your baby at the right temperature at night is to put them in multiple layers. A general rule of thumb to apply is that your baby should wear one more layer than what you would feel comfortable in.

Layers are considered much safer than big, bulky garments because they tend to be more breathable and significantly reduce the risk of overheating. Overheating poses a serious danger to your child, increasing their risk of sudden infant death syndrome.

Layers also make it much easier for you to adjust according to the baby’s temperature, as you can easily add or remove them. They are less restrictive than a bulky outfit, which may make your baby comfier throughout the night.

You might think that if you dress your baby poorly, the worst that’s likely to happen is that they’ll cry and wake you up - but often, babies won’t show many signs of distress if they’re at the wrong temperature. Becoming too cold or too hot can be very dangerous or even fatal in some situations, so it’s very important to take this seriously and be careful.

Baby sitting on lambskin rug wearing a Woolino merino wool bodysuit in blue

Baby Bodysuit, Long Sleeve, Merino Wool, Blue - $35.99

What to Wear Under a Sleep Sack Indoors

So, what should your baby wear under a sleep sack?

Fortunately, there are a number of options to choose from! If it’s cold, you may want to choose something warm that will cover your baby’s arms and legs, such as cozy footie pajamas. This will ensure that your baby’s arms and legs stay nice and warm.

Other popular options include bodysuits and footed pants, both of which are thin enough to fit under a sleep sack while ensuring you’re not adding clothing that is too thick. 

A bodysuit will keep your baby’s arms warm, while the footed pants will add an extra layer of warmth without restricting the upper body. You can also pair cool clothing with a light sleep sack, made of soft cotton for extra breathability. A footless sleep sack might be another good option for babies in hot climates.

Some parents wonder whether you can layer sleep sacks, but in general, this is not considered safe, as it’s likely to leave your baby too hot (even in cold weather), and may be too restricting. You’ll want plenty of sleep sacks so you’ll always have enough, but your baby should only wear one at a time.

Woolino’s onesies and bodysuits are all made of breathable merino wool fabric, which is ideal for maintaining the baby’s body temperature. Merino wool helps to wick moisture away from the baby’s skin, leaving it dry and warm. It also allows for plenty of air exchange with the outside world, helping to prevent overheating, while providing a pocket of warmth around the infant’s body.

On the whole, “less is better” when it comes to choosing clothing for your baby to sleep in. As long as their room is at a comfortable temperature and they have a good sleep sack and another layer or two underneath, they will generally be comfortable throughout the night.

Dressing a Baby for Sleep

So, what else do you need to know about dressing a newborn for sleep? There are quite a few considerations, not just the number of layers that you choose. It’s very important to think about the fit and comfort level of the clothes you choose.

While adults may prefer loose-fitting sleepwear, the garments you choose for your baby should be snug, with no loose parts. This is because you want to eliminate any risk of your child pulling garments off and getting tangled up in them or suffocated by them. Make sure clothing fits snugly against the body.

Woolino’s pajamas and bodysuits are both designed with this in mind, ensuring your little one will sleep soundly throughout the night. They make ideal sleepwear beneath the sleep sack, keeping your infant comfy throughout the night.

Having comfortable bedding, such as cotton crib sheets, will increase the chances of your child sleeping well throughout the night.

It’s also important to understand a few “don’ts” when it comes to figuring out how to dress a baby in a sleep sack. One of the most important is that you should never include a loose blanket in the bed until your baby is at least 12 months old. Indeed, you should avoid putting any loose fabric in the bed with your baby, including pillows, stuffed toys, etc.

Choose sleepwear that doesn’t have toggles or buttons, as there is a risk that these could be pulled off as your child gets a little older, and they may present a choking hazard. You should never put your baby to bed in a hat, even if the room is cold as this isn’t safe.

Baby laying on lambskin rug wearing a Woolino merino wool sleeper in blue stripes

What to Wear Under a Sleep Sack Outdoors

If you’re taking your baby outdoors in their sleep sack, the calculation may be a little different, as they will no longer be in a temperature-controlled environment, and may need more protection from the elements.

The “one more layer” rule is still a good one to follow, but there are a few more things to think about. 

Firstly, you need to protect your baby’s extremities with much more care. That means choosing clothing items like mittens, hats, and snug merino wool socks for their feet.

What they should wear under a sleep sack will depend heavily on the temperature, but stick to the same principle of having multiple layers. This will allow you to add or remove layers according to the environment.

It’s again important to avoid your baby overheating. Have an extra couple of layers in a bag so you can warm them up if necessary, but don’t over-dress your child, even if the weather is cool. Instead, monitor them and increase their layers only if necessary. Having a cozy blanket or a swaddle blanket can be a nice option, but never use these under the straps of a car seat, as they will make it impossible to sufficiently tighten the straps of the seat. 

Remember to protect your baby from extremes of temperature as much as possible while you’re out and about, even if they are well dressed. Sit them in the shade, face them away from the wind, and keep outdoor trips short in severe weather, especially with a newborn.


It’s not easy figuring out how to dress a baby under a sleep sack, especially for new parents. There’s often a bit of a learning curve, but if you use breathable sleepwear like Woolino’s merino options, you’ll find it much easier. 

The 4-season sleep sack has been specifically designed for year-round use regardless of the temperature, thanks to the wool’s fabulous temperature-regulating properties.

Always adjust what your baby wears underneath a sleep sack to help them stay cool when the weather is hot and to keep them snug when the weather is cold. Follow the information above and rest easy knowing that your little one is getting the best possible rest all night long.


What is the ideal room temperature for a baby, and does this affect what they should wear under a sleep sack?

The best room temperature for a baby is between 68 and 72 degrees F, and you should always aim to have their bedroom within this range. Most babies will then be comfortable in just one layer (or possibly two), plus the sleep sack. 

If the room is colder, you might want to add an additional layer, but remember that overheating is very dangerous. If the room is hot, opt for fewer layers or nothing but the diaper under the sleep sack.

How can I tell if my baby is too hot or too cold under the sleep sack?

Checking your baby’s temperature is key to managing their clothing, so look out for signs of excessive warmth or chilliness. Being too hot can lead to:

  • Flushed cheeks
  • Sweatiness
  • Heat rash
  • Fast breathing
  • Feeling hot to the touch

Being too cold might result in:

  • Lethargy/lack of energy
  • A cold red face or pale skin
  • Having very cold hands and feet
  • Having a cold chest/back of the neck (core temperature)

What types of clothes are best to wear under a sleep sack?

You can put any clothes that are well-fitted and comfortable under a sleep sack. A lot of babies wear pajamas under sleep sacks, but for a newborn, pajamas can be fiddly and a lot of work. A onesie or bodysuit may be easier until your child has a little more motor control. Whatever you choose, Woolino’s clothing has been carefully designed to meet every need and provide the baby with warmth, moisture-wicking, and good temperature regulation.

Can my baby overheat with clothing under a sleep sack?

Yes, babies are very vulnerable to overheating. You need to find a balance where your baby is warm enough, but not too hot at night. It was this challenge that drove many of Woolino’s products, especially our breathable, natural sleep sack, designed to ensure the baby is always warm enough, without the risk of becoming too hot.