Sleep Sacks: Why, when, how long to use, and sizing tips by age

Updated Jul 15, 2022
Sleep sacks

Parenting involves a lot of decisions, especially when it comes to your baby’s sleep. If you’re looking at sleep sacks and trying to choose between the lemon, stripes, or cloud patterns, we can’t help you there (they’re all pretty adorable). But if you need to figure out whether you should use a sleep sack and for how long, we can help you cross those decisions off your (mile-long) list. 


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Spoiler alert: we’re big fans of sleep sacks at Huckleberry. Sleep sacks, aka wearable blankets, provide an extra layer of warmth without restricting a baby’s arms. The standard sleep sack forms a pouch around the baby’s legs, which allows for plenty of freedom of movement (but makes it harder for a toddler to throw a leg over the side of the crib and climb out).

Will a sleep sack magically turn your baby into a champion sleeper all on its own? Probably not. However, it can safely provide warmth and comfort to your child. When used as part of a consistent sleep routine, sleep sacks can also help cue that it’s time to transition to sleep, which can translate into an easier bedtime. 

If you’re wondering why you can’t just use a blanket to keep your baby warm, here’s the answer: the American Academy of Pediatrics (AAP) recommends keeping loose blankets and unfitted sheets out of a baby’s sleep space for the first 12 months of life to reduce the risk of sleep-related injury. Sleep sacks are a safer alternative. 

You’ll want to keep size, comfort, and safety in mind. It’s a good idea to use one until at least 12 months of age since a loose blanket can increase the risk of sleep-related injury or death. Feel free to continue using a sleep sack as long as it fits well and your child seems comfortable wearing it. For instance, it’s probably time to stop using the wearable blanket if your toddler starts regularly unzipping it, taking it off, and throwing it at your head.

You can use a sleep sack from day one. However, many parents will instead opt to swaddle their newborn baby at naptime and bedtime. The window for swaddling is pretty short, though. The AAP recommends removing the swaddle as soon as your baby shows signs of starting to roll. While that’s typically between 3 - 4 months of age, sometimes it’s as early as 2 months old. 

When you’re ready to transition from the swaddle (or if you skipped it altogether), a sleep sack can be used to ensure your baby stays comfortable. It’s a safe alternative to using a sheet or blanket (which, again, is not recommended for babies under 12 months of age).

While the AAP only cautions against using blankets for infants, that doesn’t mean loose blankets are the best option for toddlers and preschoolers. Blankets tend to shift around, especially with active sleepers. Sleep sacks can help avoid all those nighttime requests for comforter adjustments. In other words, a sleep sack may help you skip the cries of, “Mommy! Daddy! Fix my blanket again!” 

If your child’s comfortable in a sleep sack, it’s fine for them to wear one until they grow out of the largest size. Traditional sleep sacks (with a sleeping bag-type pouch at the bottom) generally fit children up to 36 pounds and 40 inches long. These styles typically accommodate kids up to 2 years of age, depending on their height and weight.

Toddler-specific sleep sacks (with pants-style legs instead of the pouch) can fit bigger kids, with some brands going up to size 5T. These are a good option for older children to stay warm without the hassle and worry of their blankets slipping down.

Sizes may vary. Be sure to consult the sizing chart for your specific brand of sleep sack. 

AgeHeightWeight
Premature infants14 - 19 in (36 - 48 cm)Up to 5 lbs (2 kgs)
Newborn (0 - 3 months)19 - 23 in (48 - 58 cm)5 - 10 lbs (2 - 4.5 kgs)
Small (0 - 6 months)23 - 26 in (58 - 66 cm)10 - 18 lbs (4.5 - 8 kgs)
Medium (6 - 12 months)26 - 30 in (66 - 76 cm)16 - 24 lbs (7 - 11 kgs)
Large (12 - 18 months)30 - 35 in (76 - 89 cm)22 - 28 lbs (10 - 13 kgs)
Extra large (18 - 24 months)35 - 40 in (89 - 101 cm)26 - 36 lbs (12 - 16 kgs)
2 - 3 Years (2T - 3T)30 - 40 in (76 - 101 cm)25 - 40 lbs (11 - 18 kgs)
4 - 5 Years (4T - 5T) 40 - 46 in (101 - 117 cm)30 - 50 lbs (13.5 - 22.5 kgs)

You know it’s coming. If you’ve chosen to swaddle your baby for optimal comfort, the time will eventually come to transition out of it. There are two options here: fast or slow. 

Fast transition: You’ll need to ditch the swaddle when your baby starts to show signs of rolling (usually between 2 - 4 months old) or if they’re regularly breaking free — that loose blanket can create an unsafe sleep condition, increasing suffocation risk. At that point, it’s time for an immediate transition: bye-bye baby burrito, hello sleep sack.

Slow transition: In other cases, you can use a more gradual approach. This is a good choice for babies who haven’t shown signs of rolling but are approaching 4 months of age and/or have been fighting the swaddle. Start by leaving one or both arms out at bedtime. You can then re-swaddle fully after the first night waking. Once your baby has had 2 - 3 nights to adjust, you can move on to arms out for the entire night, then for naps, and then finally swap the swaddle for the sleep sack. 

Either way, expect some adjustment time. Whether you make a quick change or a slower one, we’ve found it’s common for babies to initially wake more once their arms are no longer confined in the swaddle. With some time, they’ll adjust to falling asleep in this new way.

Unfortunately, no. The AAP says to avoid all weighted sleep products with babies. That means no weighted swaddles, sleep sacks, sleepers, or blankets.

Consider the room temperature and how many layers you need. Generally, you’ll want to dress your baby in the same amount of layers that you’d have for sleep (or maybe one additional layer if you feel it’s needed). 

Don’t over-bundle. Overheating can cause an increased risk for sudden infant death syndrome (SIDS), so you’ll want to look out for signs that your baby’s too warm: sweating, flushed skin, or the baby’s chest feeling hot to the touch. Your infant may be comfortable in only a onesie or long-sleeved pajamas under the sleep sack, depending on the ambient temperature and thickness of the wearable blanket.

Sleep sacks for babies FAQ

Q: Why do babies like sleep sacks?

A:

Sleep sacks can safely keep babies warm and cozy during naps and night sleep. They can also be used as a consistent cue for sleep, which can make it easier for babies to transition at bedtime to sleep.

Q: Are sleep sacks safe for babies that can roll over?

A:

Yes, the AAP recommends using sleep sacks once your baby starts showing signs of rolling. Sleep sacks provide a safe alternative to swaddling or using loose blankets in the sleep space once your baby starts to roll.

Q: Can babies sleep on their stomachs in a sleep sack?

A:

Always place your baby on their back for sleep for the first 12 months of life. Babies don’t always stay on their backs, though! If your infant rolls onto their stomach, you don’t need to turn them back over as long as they’re comfortable rolling both ways (back to front and front to back).

Q: Can babies suffocate in a sleep sack?

A:

Sleep sacks are generally considered safe. To help reduce the risk of sleep-related death, be sure to place your baby to sleep on a firm, flat sleep surface with only a fitted sheet (and without loose blankets or other bedding, pillows, or soft toys).

Q: Do babies need sleep sacks in summer?

A:

Whether your baby needs a sleep sack in warmer weather depends on the ambient temperature in the bedroom. Dress your child in the same amount of layers you’d use in bed (plus one additional layer if needed). A lighter-weight sleep sack can provide a safer alternative to a loose sheet or light blanket.

Q: Do babies’ arms get cold in sleep sacks?

A:

It’s possible, depending on the room temperature. If you need long sleeves in the bedroom or your baby’s arms feel cool to the touch, then ensure your baby has long-sleeved pajamas underneath the sleep sack.

Q: My baby is having a hard time sleeping without the swaddle. Should I use a swaddle transition product?

A:

There are a number of sleep sack products designed specifically for transitioning out of the swaddle. These can be helpful for babies who wake frequently at night when their arms are free due to the Moro reflex (which typically begins to disappear starting at 12 weeks). Be sure to follow manufacturer instructions and AAP recommendations for safe sleep.

Note: The content on this site is for informational purposes only and should not replace medical advice from your doctor, pediatrician, or medical professional. If you have questions or concerns, you should contact a medical professional.