A TOG sleep sack is a wearable blanket for babies rated by its warmth level using a measurement called TOG, which stands for Thermal Overall Grade. The TOG number tells you how much heat the fabric retains: the higher the number, the warmer the sleep sack. Ratings typically range from 0.2 (very lightweight, almost no insulation) up to 3.5 (heavy winter warmth), and choosing the right one depends on the temperature of your baby’s room.
Sleep sacks replace loose blankets in the crib, which are a suffocation risk for infants. The TOG rating takes the guesswork out of how warm or cool that sleep sack actually is, so you can match it to the season and your home’s temperature.
How the TOG Scale Works
TOG measures thermal resistance, or how well a fabric traps heat between your baby’s body and the outside air. A 0.2 TOG sleep sack is barely more than a single layer of cotton, while a 3.5 TOG sack provides serious insulation for cold rooms. Most parents end up using one or two ratings throughout the year, depending on their climate and whether they run air conditioning or heat at night.
Here’s the general breakdown:
- 0.2 to 0.5 TOG: Best for warm rooms above 75°F. Think summer nights or homes without air conditioning.
- 1.0 TOG: The most versatile option, suited for room temperatures between 68°F and 75°F. This covers a typical climate-controlled home year-round.
- 2.5 TOG: Designed for cooler rooms between 61°F and 68°F. Common in colder climates during fall and winter.
- 3.5 TOG: For rooms below 61°F. This is heavy insulation and only necessary if your home gets genuinely cold at night.
If you’re unsure where to start, a 1.0 TOG is the safest bet for most households. You can always adjust clothing layers underneath rather than buying a new sack for every five-degree shift in temperature.
Why TOG Matters for Safe Sleep
Overheating is a recognized risk factor for SIDS (sudden infant death syndrome), and the TOG rating exists partly to help parents avoid it. A study from New Zealand found that excessive thermal insulation above 2 TOG, combined with placing a baby on their stomach, significantly increased SIDS risk. Research has also shown that SIDS victims tend to be found with higher thermal insulation, averaging around 2.7 TOG, especially when they were already ill.
This doesn’t mean a 2.5 TOG sack is dangerous. It means the TOG needs to match the room temperature. A 2.5 TOG sleep sack in a 72°F room is too warm. That same sack in a 63°F room is appropriate. The risk comes from mismatching warmth levels, not from any single TOG rating on its own.
What to Dress Your Baby in Underneath
The sleep sack is only one layer. What your baby wears inside it changes the total warmth, so the clothing underneath should shift with both the TOG rating and the room temperature.
- 0.2 to 0.5 TOG (75°F+): A diaper only, or a short-sleeve bodysuit at most.
- 0.5 to 1.0 TOG (72°F to 75°F): A short-sleeve bodysuit.
- 1.0 TOG (68°F to 72°F): A long-sleeve bodysuit or lightweight pajamas.
- 2.0 to 2.5 TOG (64°F to 68°F): A long-sleeve bodysuit plus warmer pajamas or a footed sleeper.
- 2.5 to 3.5 TOG (below 64°F): A warm base layer under warmer pajamas. Consider warming the room as well.
A helpful rule of thumb for the popular 1.0 TOG sack specifically: in a warm room (72°F to 75°F), pair it with just a short-sleeve bodysuit. In a comfortable room (68°F to 72°F), switch to a long-sleeve bodysuit or lightweight footie pajamas. If the room dips to 65°F to 68°F, footie pajamas or a long-sleeve bodysuit with lightweight pants work well.
How Fabric Affects Warmth
Two sleep sacks with the same TOG rating can feel quite different depending on the material. Most sleep sacks use cotton, polyester fleece, bamboo viscose, or wool, and each handles heat and moisture differently.
Cotton is breathable and widely available but doesn’t regulate temperature actively. It keeps heat in when layered but can feel clammy if a baby sweats. Polyester fleece provides warmth efficiently, which is why it’s common in higher-TOG sacks, but it doesn’t breathe as well as natural fibers.
Merino wool is the standout for temperature regulation. It creates a microclimate around the body, gaining or releasing heat as the surrounding temperature changes. That means it helps keep a baby warm when the room is cool and allows cooling when the room is warm. Some merino wool sleep sacks are marketed as “four-season” products without a fixed TOG rating because the fiber naturally adjusts. They tend to cost more, but parents who keep their homes at varying temperatures, or who don’t want to buy multiple sacks, often find the investment worthwhile.
Bamboo viscose falls somewhere between cotton and merino. It’s soft and moisture-wicking, making it popular for lighter TOG ratings in warmer climates.
How to Tell if Your Baby Is Too Warm
No chart replaces checking your actual baby. Room thermometers help, but the real test is touching your baby’s skin, specifically the chest, back of the neck, or tummy. If these areas feel hot or damp with sweat, the setup is too warm. Cool hands and feet alone are normal in infants and don’t reliably indicate that a baby is cold.
Other signs of overheating include flushed or red skin, rapid breathing, and unusual fussiness or restlessness during sleep. Heat rash, particularly around the neck, back, and underarms, is another clear signal. On the flip side, a baby who is unusually quiet or lethargic may also be too hot. If you notice any of these patterns, remove a layer or switch to a lower TOG sack and see if sleep improves.
Choosing the Right TOG for Your Home
Start by checking the overnight temperature in your baby’s room, not just the thermostat setting. A room thermometer placed near the crib gives you a reliable number. Rooms on upper floors, rooms with large windows, and rooms far from the thermostat can differ by several degrees from what your HVAC system reports.
Most parents in climate-controlled homes do well with two sleep sacks: a 1.0 TOG for the majority of the year and either a 0.5 TOG for summer or a 2.5 TOG for winter, depending on where they live. If your home stays between 68°F and 74°F year-round, a single 1.0 TOG sack with clothing adjustments underneath may be all you need.
Keep in mind that babies don’t regulate their body temperature as effectively as adults, especially in the first few months. A room that feels comfortable to you in a t-shirt and shorts is likely comfortable for a baby in a 1.0 TOG sack with a light bodysuit. When in doubt, lean slightly cooler rather than warmer. It’s easier and safer for a baby to warm up than to cool down while swaddled in an insulated sack.