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The Number That Answers the Question
Somewhere around 75°F, a lot of parents pull out whatever sleep sack is in the drawer and hope for the best. That works fine in January. In July, it can mean a sweaty, restless baby and a long night for everyone involved.
TOG — which stands for Thermal Overall Grade — is a standardized unit of measurement used by the textile industry to describe how much thermal insulation a garment provides. The higher the TOG value, the more heat the sack retains. The lower the number, the more freely heat can escape. It sounds simple, and it mostly is, but the tricky part is that the right TOG isn’t about the season outside — it’s about the temperature inside the room where your baby actually sleeps.
That distinction matters more than most parents expect. A nursery in a well-air-conditioned apartment in Phoenix in August might sit at 70°F, while an older home in the Midwest with no AC can hit 82°F by midnight. Same calendar date, very different TOG needs.
The Room-Temperature Breakdown
Here’s how the TOG scale maps to nursery temperatures, based on widely used guidelines:
Above 75°F (24°C): This is the summer threshold. At these temperatures, a 0.5 TOG sleep sack is the standard recommendation. It provides just enough of a layer to give babies the familiar snugness of a sleep sack without trapping meaningful heat. If the room climbs above 78°F and stays there, some families skip the sack entirely and dress baby in just a short-sleeve onesie — though a 0.5 TOG still tends to support better sleep by mimicking the contained feeling babies associate with bedtime.
69–75°F (21–24°C): This is where 1.0 TOG lives. It’s a light layer of insulation — appropriate for air-conditioned rooms that hover in the low-to-mid 70s. A short-sleeve bodysuit underneath is usually enough. If your nursery stays in this range all summer because you run the AC, a 1.0 TOG is probably the more practical pick.
68–75°F (20–24°C) with variability: If the temperature drops several degrees overnight — common in homes where AC cycles off — a 1.0 TOG paired with a long-sleeve layer underneath tends to cover the range. The layering principle is straightforward: the clothing worn under the sack contributes roughly 0.5 TOG equivalent of warmth, giving parents flexibility without needing a different sack for every degree of change.
Below 68°F (20°C): Once the room gets into the mid-60s, a 2.5 TOG is more appropriate. This isn’t a summer scenario for most US nurseries, but it’s worth knowing if you keep the AC set very low or have a nursery that runs cold.
One important note: these ranges are guidelines, not rules. Babies vary. Some run warm, some run cool, and a newborn’s temperature regulation is still developing. The most reliable check isn’t a thermometer — it’s feeling the back of your baby’s neck or their chest. Sweaty or flushed means too warm; cool and mottled means too cold. Hands and feet tend to run cool regardless of core temperature, so they’re not a useful signal.
Why Fabric Matters as Much as TOG Rating
A 0.5 TOG number tells you about insulation. It doesn’t tell you anything about breathability, moisture management, or how the fabric behaves when a baby sweats. In summer, those properties are where the real difference shows up.
TENCEL™ Lyocell — the fabric used across Loulou Lollipop’s sleep sack line — is derived from sustainably sourced wood pulp using a closed-loop manufacturing process that recycles over 99% of its solvents. From a functional standpoint, its fiber structure absorbs moisture and releases it quickly, which means it actively moves heat and sweat away from the skin rather than trapping it against the body. That’s a different performance profile than cotton, which absorbs moisture but dries slowly, or synthetic fabrics, which tend to block airflow altogether.
For a baby sleeping in a warm room, this matters in a specific way: a sleep sack made from a breathable, moisture-wicking fabric can feel significantly cooler at the same TOG rating compared to a denser or less breathable material. The TOG number is a standardized insulation measure, but the experience of wearing that sack is shaped by the fabric’s ability to regulate the microclimate between the garment and the skin.
Loulou Lollipop’s TENCEL™ sleep sacks are also OEKO-TEX Standard 100 certified, meaning they’ve been tested and confirmed free of harmful chemicals — a detail that carries extra weight for newborns and babies with sensitive or eczema-prone skin, who are already more reactive to heat and irritation.
The 1.0 TOG TENCEL™ sleep sack is suited for rooms in the 69–75°F range, while the 0.5 TOG muslin sleep sack is designed for warmer conditions and late spring through summer months — made from soft, breathable muslin with a sleeveless cut that keeps airflow moving.
Putting It Together: A Practical Summer Decision Tree
The question parents usually arrive at isn’t really “what is TOG” — it’s “what do I actually put on my baby tonight.” So here’s how to think through it without overthinking it.
Start with a room thermometer. Not the weather app, not a general sense of whether it’s warm — an actual reading from the nursery, taken close to bedtime. Temperatures can shift a few degrees between when you put baby down and 3 AM, especially in homes where AC cycles on and off.
If the nursery reads above 75°F: reach for the 0.5 TOG. Dress baby in a short-sleeve onesie or just a diaper underneath, depending on how warm the room feels. Watch for signs of overheating — sweaty hair, flushed cheeks, rapid breathing — and remove a layer if you see them.
If the nursery reads 69–75°F: a 1.0 TOG with a short-sleeve bodysuit is the standard starting point. If the room tends to cool down significantly overnight, a long-sleeve layer underneath gives you a small buffer without switching sacks.
If you’re unsure whether your baby runs warm or cool, start lighter. It’s easier to add a layer than to wake a sleeping baby to remove one.
And if you’re building out a sleep sack collection for the year, most families find they need two to three TOG options to cover the full range of US nursery temperatures across seasons — a lightweight option for summer warmth, a mid-weight for air-conditioned rooms and shoulder seasons, and a heavier option for winter. Loulou Lollipop’s sleep bag collection covers all three, from 0.5 TOG muslin through 1.0 and 2.5 TOG TENCEL™ — each designed to AAP safe-sleep guidelines with a sleeveless cut and two-way zipper for easy nighttime diaper changes.