What Should Baby Wear Under Swaddle or Sleep Sack?

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What should baby wear under swaddle to stay comfortable and avoid overheating? A 4-season guide to dressing your baby under a swaddle.

As a new parent, soothing a newborn can feel like an endless task.

And it’s no secret that babies aren’t the best sleepers. They wake up often for feedings, but also for comfort as they get used to life outside the womb.

When done properly, swaddling your baby for sleep provides a sense of security and comfort that’s similar to being in the womb. This is especially important for newborns who are adjusting to a new environment. And it can help them as they get used to a sleep schedule during those first couple months of life. 

Swaddles are cozy and secure, and can help babies sleep better. Which is great news.

But what should baby wear under their swaddle?

Because overheating is a common issue you want to avoid, many parents wonder what should baby wear under the swaddle for sleep.

In this case, less is usually better. Naked with a diaper, a cotton short sleeve onesie or long sleeve onesie is likely all your baby needs to feel comfortable, with a well regulated body temperature, across seasons and climates.

However, cooler winters or hotter summers may call for an extra layer of clothing, or one less.

Below, we share proper swaddling techniques and go over how to dress your baby for sleep so they’re comfy, and you don’t have to worry if they’re too hot or cold while they sleep.

What is Swaddling?

Swaddling is an age-old practice in which babies are wrapped tightly in blankets or layers of fabric to restrict their movements and mimic what it would have felt like to be in the womb.

Swaddling helps keep a baby from flailing his or her limbs when they sleep at night, which can disrupt their rest.

Other swaddling blanket benefits include longer and calmer sleep, which is also great news for sleep-deprived parents and caregivers.

While swaddling can have lots of benefits, proper swaddling techniques are important to reduce the risk of overheating or Sudden Infant Death Syndrome.

This makes many parents wonder: what should baby wear under swaddle?

Swaddle vs Sleep Sack vs Zip Up Swaddle

When it comes to swaddling newborns at night, many parents ask whether a swaddle, a swaddle sack or zip-up swaddle, or a sleep sack is better for baby’s health and safety (not to mention convenience).

Let’s go over the differences.


A swaddle is made up of one large piece of fabric, often breathable muslin, that you wrap baby in like a cozy burrito. This is the most traditional method of swaddling a baby, and has been used for generations.

However, these days there are some alternatives.

My personal favorite is a zip-up or velcro swaddle sack.

Swaddle Sack

Swaddle sacks provide the benefit of swaddles (a tight fit for baby that restricts limb movement and keeps their arms in a set position), without the hassle of actually wrapping baby up. Instead, they use zippers or velcro to keep everything in place. I prefer zippers over velcro, because it’s quieter if baby wets their diaper at night, and you need to do a sleepy time change.

Swaddle sacks also offer different arm positions. Some use a traditional arms down by baby’s side position, whereas others (which we used, and my baby loved) position baby’s hands over their head, as if they’re about to throw out some “high tens” for a job well done.

baby in a swaddle sack

Finally, swaddle sacks are designed to ensure there is no loose fabric near baby’s face, which is dangerous, and have some benefits in terms of hip dysplasia, explained below.

Sleep Sack

In addition to a swaddling blanket or swaddle sack, you can also choose a sleep sack to dress baby to sleep in.

A sleep sack is a wearable blanket that’s kind of like a sleeping bag with arm holes. They ensure baby has some mobility, but they also keep them warm, since babies can’t use blankets for safety reasons (remember: babies should always sleep in a bare crib or pack n play with no loose blankets, pillows, stuffed animals, etc.).

toddler in a sleep sack asleep in a pack n play

Whereas swaddling is meant for newborns, sleep sacks can be used for older babies, and even toddlers. Swaddling needs to stop once baby is able to roll over or move on their own within their crib, because it’s a safety issue for baby to not have control over their arms once they can move.

However, you can discontinue swaddling earlier, too, and switch to a sleep sack. That’s exactly what we did in our family – we used a zip-up swaddle sack with our daughter until she was around 3 months, then we switched to a sleep sack.

A Note About Hip Dysplasia

One of the reasons we decided to go with a swaddle sack for the first 3 months, followed by a sleep sack when our baby was a bit older, is because of the risk of hip dysplasia.

Wrapping your baby too tightly in a traditional swaddle can cause hip dysplasia, whereas many swaddle sacks are designed to avoid this issue, and some are even certified as a safe alternative by hip dysplasia organizations.

If you do choose to use a traditional swaddling blanket to get baby to sleep better, be sure you don’t wrap them up too tightly.

And ensure there’s no loose fabric near their face.

Can a Baby Get Too Hot in a Swaddle?

Babies can’t regulate their own body temperature, which is why you need to dress them properly under their swaddle.

If you’re wondering what your baby should wear under swaddle, remember that babies dressed in too many layers can overheat. Overheating can increase the risk of SIDS, and it’s important to monitor your baby throughout the night to ensure they aren’t showing signs of discomfort or being too hot.

Look for sweaty hair or head, hot feeling or sweaty neck and ears, or generally feeling warm when you touch them. Don’t use your baby’s hands or feet as an indication of whether they’re too warm, as their hands and feet are often cool to the touch, even if they are actually too warm.

Because the risk of overheating is real, it’s better to err on the side of caution, which means fewer layers and less clothing underneath the swaddle.

Often, we dressed our daughter in a diaper plus swaddle sack only. When slightly cooler weather hits, you may want to dress them in a lightweight short sleeve onesie, and in the winter a long sleeved onesie under the swaddle.

Generally speaking, you want to ensure the nursery is on the cooler side of things (around 20C to 22C or 68F to 72F), and avoid very thick, heavy clothing.

SIDS incidences tend to be higher in the winter than in the summer, which some researchers think is due to babies being dressed too warmly, and over heating while sleeping.

Signs Your Baby is Overheating

A good rule of thumb is to monitor your baby’s skin temperature to ensure he or she isn’t overheating.

The best place to feel your baby’s skin is at the base of the neck and your baby’s head. If your baby’s body is red or is accumulating moisture, this may indicate they’re at risk for overheating. Other signs that your baby may be overheating include flushed skin, rapid heartbeat, and restlessness. You’ll want to monitor your baby closely for signs they are overheating to reduce the risk of SIDS and overall discomfort. 

If your baby has overheated be sure to offer them plenty of fluids, change to lighter clothes or remove their clothes entirely, or sponge bathe your baby in lukewarm or cool water. If symptoms do not improve, contact your paediatrician or emergency.  

How to Dress Your Baby for Summer Under a Swaddle

Depending on where you live, the summer months can drastically change the temperature inside or outside your home. If you are prone to using an AC a lot when it’s hot outside, you’ll want to dress your baby appropriately, and carefully control the temperature inside the nursery.

If your home temperature gets hotter with the change in season, your baby will require less clothing.

During the summer months, the humidity in the air will also impact the room temperature.

For best results, keep a thermometer in the baby’s room or purchase a smart thermometer that allows you to see the nursery room temperature from an app.

In hotter temperatures, you should only need to dress your baby in one light layer of clothing under the swaddle at maximum, such as a short-sleeved 100 percent cotton onesie.

More likely, your baby will be comfortable mostly naked under the swaddle, wearing just a diaper.

What to Wear in the Winter Under a Swaddle

In the winter, your baby may need extra warmth. If your house is below 65F at night, a long-sleeve onesie or one-piece pajamas worn under the swaddle should be enough to keep your baby comfortable. But if your house temperature is over 65F at night, dressing your baby in a short-sleeved onesie for sleep should work fine.

Remember that a swaddled baby will feel even warmer being in a cocoon of body heat and fabric, so keep that in mind when determining the amount of clothing. 

Signs your baby may be too cold include cold nose, pale skin, or cool skin. Since baby’s hands and feet are usually quite cool, they’re not a great indicator if baby is too cold, although they certainly shouldn’t feel ice cold to the touch.

While it might be tempting to overdress your baby in the winter, a simple long-sleeved onesie and socks should keep your baby warm under the swaddle.

If your baby’s hands are cold, consider using no scratch mittens.

Avoid hats because they will trap too much heat in your baby’s body, causing his or her temperature to rise quickly. They also might slip down baby’s face in the night, too. 

Other Considerations

It’s important to remember that every baby has his or her own preferences when it comes to comfort and temperature.

Restlessness, irritability, and crying are all signs that your baby is too hot. A baby who is overly interested in feeding or cold skin may be too cold.

While swaddling is a great solution for calming your baby, nothing can replace human touch. If your baby is crying or restless for prolonged periods of time, consider rocking your baby or breastfeeding, and then put them back in their crib when they’re drowsy but still alert, so as to not fall asleep while being held.

Some babies may prefer extra wiggle room and have their arms free. It’s up to you to decide what works for your baby.

Usually, a baby at a comfortable temperature will be calm and more receptive to swaddling. Be sure to swaddle your baby snuggly but not too much. A general rule of thumb is that you should be able to fit two to three fingers between the blanket and your baby’s chest. 

Room Temperature

Keeping the room temperature at between 68 and 72F is ideal for ensuring that your baby doesn’t overheat or get too cold. In the fall and spring, it’s common for temperatures to fluctuate inside and outside the home. Keeping your home at a consistent temperature will help your baby stay calm and feel soothed.

Babies overheat and have fevers around 100.4 F (38C). Keep in mind that normal baby body temperatures can vary throughout the day.

If you still have doubts, remember the right temperature for babies is usually the same as adults wearing light clothing. 

Material and Thickness of the Swaddle

Most popular swaddles are made out of a 100 percent cotton blanket, lightweight cotton muslin, or microfleece.

Swaddles should come with a TOG (Thermal Overall Grade) rating when you purchase them, which provides a score for the best use of the fabric. A rating of 0.5 TOG for example is ideal for warmer seasons while a rating of 3 TOG is best for keeping them warm enough in colder temperatures.

We used a 0.5 TOG for our daughter in summer, and she wore only a diaper underneath (we didn’t use air conditioning, and lived in a hot climate). In winter, we switched to a sleep sack due to her age. We used a 2.5 TOG sack, with light pajamas and a diaper underneath.

There are many swaddling blanket options with natural fibers in addition to cottons such as cashmere, silk, wool, and hemp. Natural fiber is best for absorbing moisture or sweat when it’s hot and provide more insulation when it’s cold. These fabrics also differ in weight. A cotton blanket, for example, may feel lighter than a microfleece. 

How Your Baby Looks & Feels

As a parent, no one knows better than you how your baby is responding to swaddling. If your baby is flush, restless, or looks uncomfortable, use your instincts to adjust.

Overheating symptoms can also include red skin, sweating, and rapid heartbeat.

While swaddling might work for some babies, others may calm down better through skin-to-skin contact or babywearing. Your baby should be able to sleep comfortably on his or her back. If your baby is getting old enough to flip over, it is time to stop swaddling or transition to a sleep sack. 

Final Thoughts on Dressing Your Baby for Sleep with a Swaddle

By selecting the right fabric and being mindful of temperature changes, you can learn to swaddle your baby for sleep the right way and dress your baby in the right outfit. While overheating is a concern because it puts babies at risk for SIDS and other health issues, monitoring your baby’s look and feel will help you determine how much clothing he or she needs under the swaddle. 

At the end of the day, trust yourself to know what your baby needs to rest peacefully. The more you practice swaddling and using a sleep sack, the more confident you will become. Seeing your baby sleeping peacefully will be all the reassurance you need to know that you are swaddling with success. 

If you found this article on what should baby wear under swaddle useful, feel free to share it with other parents you know who are interested in techniques to swaddle their baby.

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