What your baby wears under a sleep sack depends on the room temperature and the thickness of the sack itself. The general rule: dress your baby in one layer more than what you’d wear comfortably in the same room. In most homes kept between 68 and 72°F, that means a long-sleeve bodysuit or lightweight footed pajamas under a medium-weight sleep sack.
Room Temperature Sets the Starting Point
The ideal nursery temperature for infant sleep falls between 68 and 72°F (20 to 22°C). That range is your anchor for choosing layers. If your home runs warmer or cooler, you’ll adjust both the sleep sack weight and what goes underneath it.
A quick way to gauge: if you’re comfortable in a t-shirt, your baby needs that same t-shirt plus one layer on top. If you’d want a light sweater, your baby likely needs a warmer base layer plus a thicker sleep sack.
How TOG Ratings Work
Sleep sacks are rated by TOG, a measure of thermal resistance. The higher the TOG number, the warmer the sack. Here’s how the ratings line up with room temperature:
- 0.2 TOG: 75°F to 81°F (very warm rooms, summer)
- 1.0 TOG: 68°F to 75°F (typical indoor temperature)
- 1.5 TOG: 64°F to 72°F (slightly cool rooms)
- 2.5 TOG: 61°F to 68°F (cool rooms, winter)
- 3.5 TOG: below 61°F (cold rooms)
Most families in climate-controlled homes will use a 1.0 TOG sack for the majority of the year. If you only buy one sleep sack, that’s the most versatile choice.
What to Pair With Each TOG
These combinations give you a practical starting point. You can fine-tune based on how your baby feels overnight.
- 75°F and above (0.2 to 0.5 TOG): Diaper only, or a short-sleeve bodysuit at most.
- 72 to 75°F (0.5 to 1.0 TOG): Short-sleeve bodysuit.
- 68 to 72°F (1.0 TOG): Long-sleeve bodysuit or lightweight footed pajamas.
- 64 to 68°F (2.0 to 2.5 TOG): Long-sleeve bodysuit plus warmer pajamas or a footed sleeper.
- Below 64°F (2.5 to 3.5 TOG): A warm base layer plus warmer pajamas. Also consider heating the room closer to the recommended range.
For the most common scenario, a 1.0 TOG sack in a room around 68 to 72°F, a long-sleeve bodysuit or a thin cotton footie is all your baby needs. If the room is on the warmer end of that range, swap to a short-sleeve bodysuit.
What to Avoid Under a Sleep Sack
Clothing with hoods, drawstrings, or loose fabric is a suffocation risk. If fabric bunches up and migrates toward your baby’s face or neck, it can obstruct their airway. Stick with fitted bodysuits, snug pajamas, or footed sleepers that stay in place.
Avoid adding hats for sleep. Babies regulate a significant amount of body heat through their heads, and a hat can cause overheating. Mittens or socks are fine if your baby’s hands and feet run cold, but they aren’t necessary for most infants in a properly heated room. Never use a weighted sleep sack, weighted swaddle, or any weighted object on or near your baby during sleep.
How to Check if Your Baby Is Too Warm
Overheating raises the risk of SIDS, so erring slightly cool is safer than slightly warm. The best place to check your baby’s temperature is the back of their neck or their chest, not their hands or feet (which tend to feel cooler than the rest of the body).
Signs your baby is too warm include flushed or red skin, sweating or damp hair, restlessness, and skin that feels hot to the touch on the chest or back. Some babies overheat without sweating, so don’t rely on sweat alone as your indicator. If your baby’s chest feels warm and clammy, remove a layer or switch to a lower TOG sack.
Fabric Matters
Both the sleep sack and the clothing underneath work best in breathable, natural fabrics. Organic cotton and bamboo are the two most common options for infant sleepwear, and both allow airflow and wick moisture away from the skin. Bamboo tends to be slightly better at temperature regulation, staying cool in warmth and retaining heat in cooler conditions. Cotton is durable, easy to wash, and widely available.
If your baby has eczema or sensitive skin, either fabric is a good choice. Both are naturally hypoallergenic and reduce moisture buildup against the skin. Synthetic fabrics like polyester trap more heat and can cause sweating, which makes temperature regulation harder.
Transitioning From a Swaddle
If your baby is still in a swaddle, you’ll need to switch to a sleep sack as soon as they show any signs of trying to roll over. This typically happens around 2 to 4 months. Once your baby can roll, having their arms compressed in a swaddle prevents them from pushing up or repositioning, which creates a suffocation risk.
The transition can be bumpy for a few nights. Some families find it helps to use a transitional sack that leaves the arms free but still provides a snug feeling around the torso. The clothing underneath stays the same: a bodysuit or lightweight pajamas chosen for the room temperature and the sack’s TOG rating.
A Simple Decision Framework
If you want to skip the charts and keep it straightforward, here’s the simplest approach: check what you’d wear to bed comfortably, then add one thin layer for your baby. In a room that feels comfortable to you in a t-shirt and light blanket, dress your baby in a long-sleeve onesie and a 1.0 TOG sack. Touch their chest before you go to bed. If it’s warm but not hot or sweaty, you’ve nailed it. If it’s cool, add a layer next time. If it’s damp, remove one.
Every baby runs a little different. Some sleep warm and do best in just a diaper under a light sack, while others need a footed sleeper even in a mild room. The charts are a starting point, but your baby’s chest temperature overnight is the real answer.