baby in crib wearing leopard print sleep sack

What Should My Baby Wear Under a Sleep Sack?

By: Brooke Hamilton


Between rocking, singing lullabies, and cuddling, preparing for bedtime with your newborn is a precious and soothing experience. But when it comes to dressing your baby for bed, you might be left wondering: what should a newborn wear under a sleep sack?

Sleep sacks are made to keep babies warm and cozy overnight. It’s one of the must-haves on every baby essentials checklist. When it comes to choosing pajamas for underneath a sleep sack, you want your baby to stay cozy and warm, but you don’t want them to overheat or become uncomfortable.

So, choosing the best under-sleep-sack outfit for your infant will depend on several factors, including fabric, season, and age. Read on for our guide on how to choose the safest, most comfortable, and most adorable pajamas for your little one.

Best Fabrics for Under Sleep Sacks

When choosing an under-sleep-sack pajama outfit for your newborn, the first thing to consider is fabric. This is because different materials offer different levels of warmth, comfort, and breathability.

Our favorite options for babies are bamboo and cotton.

Viscose from Bamboo

Viscose from bamboo is made by soaking and shredding bamboo stalks into pulp, spinning the pulp into thread, and using that thread to weave fabric.

This seasonless fabric is your best bet to wear under a sleep sack because it is breathable, while keeping your baby warm and cozy. Viscose from bamboo offers a heavenly, buttery-soft texture and is gentle on sensitive skin. Not to mention, it wicks moisture away easily and stretches as your baby grows, so you can use each pajama set for longer.

Ultimately, viscose from bamboo is a no-brainer if you’re looking for a soft, seasonless fabric that’s gentle on baby skin.

Organic Cotton

Organic cotton is another common fabric used in baby pajamas sets. Grown without pesticides, this fabric is considered eco-friendly and is relatively gentle on baby skin. However, since it’s not quite as soft as viscose from bamboo and contains fibers that expand and contract, cotton can still be irritating on delicate skin.

While cotton doesn't stretch like viscose from bamboo, it does wick sweat well and helps regulate your newborn’s body heat naturally. While not quite the catch viscose from bamboo is in terms of pajamas, it certainly serves as a close runner-up.

Temperature Guidelines for Baby Clothing

Temperature is paramount when considering what a baby should wear under a sleep sack. Your number-one goal should be to help regulate your baby’s body temperature, keeping them warm through the night without causing them to overheat. This is why a “seasonless” fabric or medium-weight pajamas are likely your best bet. You may also consider rotating pajamas according to the season and the temperature in your home.

Age-Appropriate Sleepwear Choices

When shopping for baby girl pajamas or baby boy pajamas , making age-appropriate choices can make all the difference when it comes to a comfortable and safe sleep.

In general, babies under 12 months old should be dressed in one extra layer than you’d dress yourself. And before they start showing signs of rolling over, babies can nap or sleep with a swaddle. As they get older and start attempting to roll over, most parents transition their baby to a sleep sack. When you enter toddlerhood and potty training, footy pajamas or one-pieces may be too difficult to take off in a pinch, so two-piece sets make more sense. Learn more about how to dress your baby for sleep.

Ensuring Baby’s Comfort and Safety

Choosing the best under-sleep-sack pajamas for your newborn comes down to comfort and safety. You want your baby’s temperature to stay regulated overnight, their PJs to be soft and gentle on their skin, and their sleep situation to be cozy and secure without becoming restrictive.

Choosing the right fabric will help you figure out how to keep your baby warm at night, while choosing an age-appropriate cut will help baby sleep comfortably and safely.

Stock up on Under-Sleep-Sack Pajamas with Posh Peanut

If you’re looking for high-quality, comfortable, and adorable sleep options for your baby, Posh Peanut has you covered. All of our swaddle options, sleep sacks, one-pieces, and baby clothes are made from super soft and stretchy viscose from bamboo. 

Plus, Posh Peanut pajamas come in a wide variety of adorable colors and patterns, and we even have adult sets to match, so you and your little one can both sleep in comfort and style.


1. Middle Georgia State University. Is the bamboo craze here to stay?

2. National Library of Medicine. Fabrics for atopic dermatitis.,that%20can%20irritate%20delicate%20skin.