Babies should sleep in one layer more than what you’d wear to be comfortable in the same room. For most homes, that means a onesie or lightweight pajamas under a wearable blanket (sleep sack), with the room set between 68°F and 72°F. No hats, no loose blankets, no extra bedding. Getting this right matters because overheating is a known risk factor for SIDS, and babies can’t regulate their body temperature the way adults can.
The One-Layer Rule
The simplest guideline for dressing your baby at bedtime: no more than one additional layer compared to what an adult would need to feel comfortable. If you’re fine in a T-shirt, your baby does well in a onesie plus a lightweight sleep sack. If you need a sweater, your baby might need footed pajamas plus a slightly warmer sleep sack. This rule scales naturally with the seasons and prevents the most common mistake parents make, which is overdressing.
Room Temperature and What to Pair With It
The Lullaby Trust recommends keeping your baby’s room between 61°F and 68°F (16°C to 20°C) for the safest sleep. Many U.S. households run a bit warmer than that, closer to 68°F to 72°F, which is still fine as long as you adjust clothing accordingly.
Sleep sacks are rated using a TOG system, which measures thermal resistance. Here’s how to match the sack to your room:
- 75°F to 81°F: A 0.2 TOG sleep sack (essentially a single layer of muslin) over just a diaper or a short-sleeve onesie.
- 68°F to 75°F: A 1.0 TOG sleep sack over a short-sleeve onesie or lightweight pajamas.
- 64°F to 72°F: A 1.5 TOG sleep sack over a long-sleeve onesie.
- 61°F to 68°F: A 2.5 TOG sleep sack over footed pajamas or a long-sleeve onesie with pants.
- Below 61°F: A 3.5 TOG sleep sack over warm pajamas.
These ranges overlap intentionally. A baby who runs warm might do better at the lighter end, while a baby who tends to feel cold can go heavier within the same temperature range. You’ll learn your baby’s pattern quickly.
Why No Hats, Blankets, or Extras
Babies release a significant amount of body heat through their heads. Covering the head indoors traps that heat and makes it harder for your baby to cool down. Hats also pose a suffocation risk because they can slide over the face during sleep. The same logic applies to loose blankets, pillows, stuffed animals, and any other soft items in the crib. The safest crib has a firm, flat mattress with a fitted sheet and nothing else.
Blankets are generally considered safe to introduce after your baby’s first birthday, though many parents continue using sleep sacks well past that point because they work well and remove the temptation to add loose bedding.
Swaddles, Sleep Sacks, and Footed Pajamas
Swaddles work well for newborns because they quiet the startle reflex and help babies settle. But you need to stop swaddling as soon as your baby shows any signs of rolling over. Those signs include pushing up during tummy time, lifting and flopping their legs to the side, or escaping from the swaddle. For most babies, this happens around 2 to 4 months. Once you see it, switch immediately to a sleep sack with arms free.
Sleep sacks are the go-to from that point forward. They keep your baby warm without loose fabric, and because they restrict leg movement slightly, they can also discourage early attempts to climb out of the crib. Some parents use them well past 16 months. The main reason to stop is if your baby is standing, cruising, or walking in the crib and tripping on the sack. At that point, footed pajamas alone (or a walker-style sleep suit with open legs) are a better choice.
Footed pajamas are the simplest option for babies who don’t need the extra warmth of a sleep sack. They cover the whole body in one piece, keep feet warm without socks (which can come off and become a hazard), and work well for babies who sleep warm. On cooler nights, you can layer footed pajamas under a sleep sack.
How to Check If Your Baby Is Too Warm
Touch the back of your baby’s neck or their chest. These areas give you a more accurate read than hands or feet, which tend to run cool naturally. If the skin feels hot, sweaty, or damp, your baby is overdressed. Other signs of overheating include flushed or red skin, damp hair, fussiness, and unusual lethargy.
Babies can overheat without sweating, so don’t rely on sweat alone as your signal. If your baby’s chest feels warm and their skin looks flushed, remove a layer or switch to a lower-TOG sleep sack. It’s always safer to dress slightly cool and add a layer than to start too warm.
Choosing the Right Fabric
Cotton and bamboo are the two best fabric choices for baby sleepwear. Both are breathable, allow air to circulate, and wick moisture away from skin. Cotton is the classic choice: soft, durable, and widely available. Bamboo has a slight edge for temperature regulation because it adapts well to both warm and cool conditions, and it tends to feel silkier against skin. If your baby has eczema or sensitive skin, bamboo is particularly worth trying because its natural properties reduce irritation and help keep skin dry.
Avoid polyester and synthetic blends when possible. They trap heat, don’t breathe as well, and can contribute to overheating. If your baby’s pajamas are synthetic, the U.S. Consumer Product Safety Commission requires that children’s sleepwear either be flame-resistant or fit snugly. Snug-fit sleepwear is the standard for most baby pajamas you’ll find in stores. Loose, flowing garments that aren’t flame-resistant don’t meet safety standards for sleepwear, so check labels if you’re repurposing daytime clothes for bedtime.
Putting It All Together
For a newborn in a 70°F room, a typical setup is a short-sleeve onesie, a 1.0 TOG sleep sack, and a swaddle if they’re not yet rolling. For a 6-month-old in the same room, swap the swaddle for an arms-free sleep sack over a onesie. For a toddler who’s mobile in the crib, footed pajamas alone may be enough.
The crib itself should have only a firm mattress with a fitted sheet. Place your baby on their back for every sleep, including naps. Offering a pacifier at bedtime also reduces SIDS risk, even if it falls out after your baby falls asleep. These basics, combined with appropriate clothing layers and a comfortable room temperature, create the safest possible sleep environment.