Babies sleep safest and most comfortably in fitted sleepwear with no loose blankets, and the right combination depends almost entirely on how warm the room is. The ideal nursery temperature is 68 to 72°F (20 to 22°C), and at that range, a long-sleeved onesie under a medium-weight sleep sack is a solid starting point. From there, you adjust up or down based on the thermometer and your baby’s cues.
The One-Layer Rule
The simplest guideline for dressing a baby for sleep: put them in no more than one additional layer than you’d need to be comfortable in the same room. If you’d sleep in a t-shirt, your baby does well in a onesie plus a light sleep sack. If you’d want a blanket, they might need a warmer sleep sack or an extra layer underneath. Babies can’t pull off blankets or kick them back on, so their clothing is their only thermostat.
What to Wear at Every Temperature
Room temperature is the single biggest factor. A thermometer in the nursery takes the guesswork out of it. Here’s what works at each range:
- Above 80°F (27°C): A short-sleeved onesie or just a diaper. If using a sleep sack, choose the thinnest option available (0.2 TOG).
- 73 to 79°F (23 to 26°C): A short-sleeved onesie with a lightweight sleep sack (0.5 TOG), or a long-sleeved onesie alone.
- 68 to 73°F (21 to 23°C): A long-sleeved onesie under a standard sleep sack (1.0 TOG). This is the sweet spot most nurseries land in.
- 61 to 68°F (16 to 20°C): A long-sleeved onesie or footed pajamas under a warm sleep sack (2.5 TOG).
- Below 60°F (16°C): Footed pajamas with a heavy sleep sack (3.5 TOG). Adding a bodysuit layer underneath is reasonable here.
TOG stands for Thermal Overall Grade, a rating printed on most sleep sacks that tells you how much warmth the fabric provides. Higher numbers mean more insulation. Matching the TOG to your room temperature and adjusting what goes underneath is the most reliable system for getting layers right.
Swaddles, Sleep Sacks, and When to Switch
Newborns often sleep best swaddled, with arms snug against their body. A swaddle calms the startle reflex, that sudden arm-flinging motion that wakes babies up. But swaddling has a hard expiration date: you must stop as soon as your baby shows any sign of rolling over. For most babies, that happens between 2 and 6 months, though some start as early as 8 weeks.
Signs it’s time to transition include rolling during playtime, pushing up on their hands during tummy time, lifting their legs and flopping them to the side, or consistently breaking free of the swaddle. Once you see any of these, switch to a sleep sack with arms free. A baby who rolls while swaddled faces a serious suffocation risk.
Sleep sacks work from the newborn stage all the way through toddlerhood. They’re essentially wearable blankets that zip on and stay in place, which means no loose fabric in the crib. They come in every TOG rating, so you can use them year-round.
Choosing the Right Fabric
Cotton is the most common sleepwear fabric, and for good reason. It’s soft, breathable, affordable, and widely available in organic options. The one downside is that cotton absorbs moisture and holds it against the skin rather than wicking it away, which can feel clammy in humid conditions or if your baby sweats heavily.
Bamboo fabric is roughly 20% more breathable than cotton and naturally wicks moisture, staying about 3 degrees cooler to the touch. The fibers are smooth and round, which makes bamboo a particularly good choice for babies with eczema or sensitive skin. It also regulates temperature across seasons, keeping babies cooler in summer and warmer in winter. The tradeoff is cost: bamboo sleepwear runs noticeably more expensive per piece.
For most families, either cotton or bamboo works well. The layer count and TOG rating matter more than the specific fabric. Avoid fleece or polyester in warm rooms, since synthetic fabrics trap heat.
How to Tell If Your Baby Is Too Hot
Overheating is a bigger concern than being slightly cool. Babies who are too warm sleep poorly, and excessive heat is a known risk factor for sleep-related dangers. Since babies can’t tell you they’re uncomfortable, you need to check.
The most reliable spot to feel is the back of your baby’s neck or their chest. If the skin there feels hot or damp with sweat, they’re overdressed. Hands and feet run naturally cooler in babies, so cold fingers don’t necessarily mean they need more layers. Check the neck and torso instead.
Other signs of overheating include flushed or red skin, heat rash (tiny red bumps, especially around the neck, back, and underarms), rapid breathing, unusual fussiness, or the opposite, unexpected lethargy. A body temperature above 100.4°F (38°C) that isn’t explained by illness can also point to overheating. If you notice any of these, remove a layer, turn down the thermostat, or switch to a lower-TOG sleep sack.
Dressing a Sick Baby for Sleep
When your baby has a fever, resist the instinct to bundle them up. A sick baby is already running warmer than usual, and extra layers can trap that heat and push their temperature higher. Dress them for the room temperature as you normally would. If anything, you may need to remove a layer to keep them comfortable. The Lullaby Trust, a leading safe-sleep organization, specifically warns against wrapping an unwell baby in more than their usual amount of clothing.
What to Avoid in the Crib
No loose blankets, pillows, stuffed animals, or sleep positioners belong in the crib with a baby. This applies through at least the first 12 months. Loose bedding is a suffocation hazard, which is exactly why sleep sacks exist as an alternative. Hats should also come off for sleep, since babies release excess heat through their heads, and a hat blocks that natural cooling system.
Sleepwear should fit snugly or be specifically designed for sleep. Loose, flowing fabric or clothing with hoods, drawstrings, or ties can bunch around the face. Footed pajamas, onesies, and properly sized sleep sacks are the safest options.