Dress your baby in one layer more than you’d comfortably wear to sleep in the same room. That single rule, recommended by the American Academy of Pediatrics, covers most situations. But the details matter: the right fabric, the right warmth rating, and knowing when to skip the swaddle can make the difference between a baby who sleeps safely and one who overheats or kicks free of loose bedding.
The One-Layer-More Rule
If you’d sleep comfortably in a t-shirt in your baby’s room, your baby needs a onesie plus one layer on top of that, like a lightweight sleep sack. If you’d want long sleeves and pants, your baby needs that plus a warmer wearable blanket. The AAP specifically recommends layers of clothing over loose blankets, which pose a suffocation and entrapment risk. Wearable blankets (sleep sacks) are a safe alternative.
This approach works because babies lose heat faster than adults due to their higher surface-area-to-body-weight ratio, but they also can’t kick off covers or tell you they’re too warm. Layering lets you add or remove warmth without introducing anything loose into the crib.
Using TOG Ratings to Pick the Right Sleep Sack
Most quality sleep sacks list a TOG rating, which measures thermal resistance. Higher TOG means more warmth. Here’s how to match the rating to your room temperature:
- 75°F to 81°F (warm room): 0.2 TOG, a very thin muslin layer. Underneath, a short-sleeve onesie or just a diaper.
- 68°F to 75°F (typical room): 1.0 TOG. A short-sleeve or long-sleeve bodysuit underneath.
- 64°F to 72°F (cooler room): 1.5 TOG. Long-sleeve pajamas underneath.
- 61°F to 68°F (cold room): 2.5 TOG. Long-sleeve pajamas plus a bodysuit layer.
- Below 61°F: 3.5 TOG. Full pajamas with a base layer underneath.
These ranges overlap intentionally. Every baby runs a little warmer or cooler, so you’ll need to check your baby’s actual temperature (more on that below) and adjust. A room thermometer in the nursery takes the guesswork out of choosing the right combination each night.
Best Fabrics for Baby Sleepwear
Cotton is the most popular choice for good reason. It breathes well, absorbs moisture if your baby sweats, and holds up to frequent washing. The tradeoff is that it can shrink in hot water and takes longer to dry than synthetics. For everyday sleepwear, 100% cotton or cotton blends work well in mild to warm climates.
Bamboo viscose has become a favorite for parents who want something softer. It regulates temperature effectively, wicks moisture, and feels lightweight against skin. It is more delicate than cotton, so it needs gentle wash cycles and low-heat drying to maintain its texture over time.
Merino wool is worth considering for colder climates. Fine merino is soft enough for babies and excels at temperature regulation, keeping babies warm without trapping excess heat. It does require gentle washing, and quality matters: cheap wool can feel scratchy and irritate sensitive skin.
Synthetic fabrics like polyester are durable and inexpensive but tend to trap heat and moisture against the skin. If your baby runs warm or sweats during sleep, natural fibers are generally a better bet.
Swaddling: When to Start and Stop
Swaddling works well for newborns because it dampens the startle reflex, that involuntary arm-flinging motion that wakes babies from sleep. A properly wrapped swaddle keeps arms snug against the body without restricting the hips.
The critical safety rule: stop swaddling the moment your baby shows any sign of rolling over. For most babies, this happens between 3 and 4 months, though some start earlier. By 4 to 6 months, the startle reflex naturally fades, which also removes the main reason for swaddling in the first place. A swaddled baby who rolls onto their stomach cannot use their arms to push up or reposition, creating a suffocation risk.
Transitional products with arms-out designs can help bridge the gap. By 6 months, most babies sleep comfortably in a standard sleep sack with both arms free.
How to Check if Your Baby Is Too Warm
Don’t rely on your baby’s hands or feet to gauge temperature. Those extremities are often cool even when a baby is perfectly comfortable. Instead, place your hand on your baby’s chest or the back of their neck. The skin there should feel warm but not hot or damp.
Signs of overheating include flushed cheeks, sweaty or damp hair, rapid breathing, and restless sleep. Overheating is a known risk factor for SIDS. Research has found that excess clothing and bedding insulation is a primary cause of dangerous heat buildup in infants. When a baby overheats, their body’s ability to wake itself up and resume normal breathing is impaired. This is why erring on the side of slightly cool is safer than slightly warm.
No Hats Indoors During Sleep
Babies lose excess heat through their heads, which is actually a feature, not a problem. Covering a baby’s head during indoor sleep blocks that cooling mechanism and raises the risk of overheating. A study of NICU infants found that removing hats during open-crib sleep did not increase the risk of getting too cold, and it eliminated an unnecessary overheating and suffocation hazard. Unless your baby is outdoors in cold weather, skip the hat at bedtime.
What to Dress a Sick Baby In
When your baby has a fever, resist the instinct to bundle them up. A fever means their body temperature is already elevated, so adding extra layers traps heat and can push their temperature higher. Dress a feverish baby for the room temperature as you normally would, or even slightly lighter. The Lullaby Trust advises dressing sick babies for their environment rather than adding warmth to “sweat out” the illness, which doesn’t help and can be dangerous.
Sleepwear Safety Standards
In the United States, children’s sleepwear must either be flame-resistant or fit snugly against the body. Snug-fit pajamas are the more common option for babies because tight-fitting fabric is less likely to catch fire than loose, flowing material. This is why baby pajamas tend to run smaller and tighter than daytime clothing. Look for labels that say “snug-fitting” or “must be snug-fitting for child’s safety.”
Pure cotton, silk, and rayon fabrics typically fail flame-resistance testing on their own, so sleepwear made from these materials is usually sold in snug-fit designs rather than treated with chemical flame retardants. If you prefer natural fibers without added chemicals, snug-fit is the way to go. Infant garments sized for the youngest babies follow a separate, less restrictive flammability standard, so you’ll see more variety in styles and fabrics for newborn sleepwear.