Quick answer
Start with the room, not the outfit. The Lullaby Trust recommends keeping a baby's room between 16 and 20°C, then dressing for that number: roughly a 1.0 TOG sleeping bag with a long-sleeve bodysuit at 20–22°C, and 2.0–2.5 TOG with warmer pyjamas at 18–20°C. Breathable cotton, one layer fewer than you think, and a hand on the back of the neck to check — that's most of it.
Bedtime is the one moment of the day where getting clothing wrong has consequences beyond comfort. Too many layers and a baby overheats; too few and they wake at 4am. And unlike daytime dressing, you can't see how it's going — you're guessing from the other side of a door.
The good news is that sleepwear is one of the few areas of kids' clothing where there is genuinely clear, evidence-based guidance. Here's what it says, translated into something you can act on tonight.
Set the room first, then dress for it
The Lullaby Trust's safer sleep guidance puts the ideal room temperature at 16–20°C, and recommends using a room thermometer rather than judging by how the room feels to you — adults are usually wearing more, moving more, and standing at a different height than a sleeping baby.
That range matters because overheating is linked to an increased risk of SIDS. Once the room is stable, the clothing question becomes simple arithmetic rather than a nightly guess. Dutch bedrooms in an old canal-house or a poorly insulated upstairs room can swing several degrees between October and February, so it's worth re-checking seasonally rather than setting one routine in September and forgetting it.
What TOG actually means
TOG stands for Thermal Overall Grade — a measure of how well a fabric traps warm air. It's not a quality rating. A 0.5 TOG sleeping bag isn't worse than a 2.5; it's for a warmer room. Lower number, lighter layer.
Here's the general guidance most sleeping-bag makers converge on, converted to Celsius:
| Room temperature | Sleeping bag TOG | What to wear underneath |
|---|---|---|
| 24°C and above | 0.2–0.5 | Nappy only, or a short-sleeve bodysuit |
| 22–24°C | 0.5–1.0 | Short-sleeve bodysuit |
| 20–22°C | 1.0 | Long-sleeve bodysuit or lightweight pyjamas |
| 18–20°C | 2.0–2.5 | Long-sleeve bodysuit plus warmer pyjamas |
| Below 18°C | 2.5–3.5 | Warm base layer plus pyjamas |
Two rules that come up again and again in this guidance, and are worth memorising:
- When in doubt, go lower. If your room sits between two bands, choose the lighter TOG. You can add a thin layer underneath; you can't easily undo overheating.
- Never stack sleeping bags. TOG ratings assume a single bag. Doubling up traps air between layers and pushes the warmth well past what the label says. The Lullaby Trust makes the same point about blankets — one blanket folded over counts as two.
Why the fabric changes the maths
TOG measures insulation, not breathability — and those are different things. A synthetic fleece sleepsuit and a cotton one can hold similar warmth while behaving completely differently once a child gets warm: cotton absorbs moisture and lets it move away from the skin, while many synthetics hold it against the body. That's the difference between a child who runs warm and settles, and one who wakes up clammy at 2am.
It's also why "cotton on cotton" — cotton pyjamas under a cotton sleeping bag — is the combination most sleep guides recommend. For kids with eczema or sensitive skin, soft organic cotton earns its place twice over: it's the layer in contact with skin for ten to twelve hours, longer than anything they wear during the day.
The safety label most parents never read
Nightwear is regulated differently from ordinary clothing. In Europe, EN 14878 sets requirements for the burning behaviour of children's nightwear — pyjamas, nightdresses, dressing gowns and similar garments — specifying limits on flash flame and how quickly a flame may spread across the fabric.
In the US, the rules take a different route: children's sleepwear from 9 months to size 14 must either be treated to be flame-resistant or be snug-fitting. That's why American cotton pyjamas carry a yellow tag reading "For child's safety, garment should fit snugly. This garment is not flame resistant." Snug garments leave little air between fabric and skin, so there's less oxygen to feed a flame — and the CPSC has said it found no burn injuries associated with snug-fitting garments.
The practical takeaway for a European wardrobe: an untreated, closely-fitting cotton pyjama is a well-established safe choice, and a loose adult T-shirt used as a nightshirt is the option regulators have repeatedly warned against. It's also a reason we cut our sleep-adjacent pieces differently from our oversized daywear — roomy is wonderful at the playground, less so in bed.
From sleeping bag to pyjamas: how the stages shift
There's no fixed age for leaving the sleeping bag behind. Many families stop around a child's first birthday, when loose bedding becomes safe; plenty continue to three or four, because a bag stays put in a way a duvet doesn't. The usual signal to move on is behavioural rather than developmental — a toddler who unzips it nightly, or climbs out of a floor bed, is telling you something.
What replaces it depends on the child. Footed pyjamas suit restless sleepers who kick everything off. Two-piece sets suit toddlers who are potty training or who want to manage their own clothes, which dovetails neatly with the independent-dressing stage. Whichever way you go, the room temperature stays the anchor: same thermometer, same touch test, fewer zips.
FAQ
How do I tell if my baby is too warm?
Feel the chest or the back of the neck. It should be warm but not hot or clammy — the Lullaby Trust names clammy skin at the chest or back of the neck as a sign a baby is getting too hot. Hands and feet often feel cool even when a baby is perfectly comfortable, so they're not a reliable test.
What room temperature should my child's bedroom be?
16–20°C is the range the Lullaby Trust recommends, measured with a room thermometer rather than judged by feel. If your house runs colder overnight in winter, adjust the sleeping bag TOG rather than adding blankets.
Can I put two sleeping bags on in a cold room?
No. TOG ratings assume one bag worn on its own, and layering them traps air in a way that can push warmth well beyond the label. Use a single higher-TOG bag, or add a light layer of clothing underneath instead.
Should my toddler wear socks to bed?
There's no strong evidence either way, and cool feet on their own aren't a sign a child is cold. If your room is at the lower end of the range, footed pyjamas or a slightly warmer sleeping bag are a tidier solution than loose socks, which tend to end up somewhere in the cot by morning.
Why are my child's pyjamas labelled "not flame resistant"?
That label appears on snug-fitting cotton sleepwear sold under US rules, which allow a garment to be either flame-resistant or close-fitting. It signals that the fabric hasn't been chemically treated — the safety comes from the fit. European nightwear is covered by EN 14878, which sets burning-behaviour requirements for children's nightwear.
When should we stop using a sleeping bag?
Whenever it stops working for your child. Some families transition around age one, others keep going to three or four. Frequent unzipping, complaints, or a move to a proper bed are the usual cues.
Keep exploring
- Dressing Kids with Eczema or Sensory Sensitivities: A Fabric & Fit Guide
- Cotton, Linen, or Bamboo? The Best Fabrics for Kids in Summer Heat
- Clothes Kids Can Put On Themselves: A Guide to Independent Dressing (Ages 2–5)
- How Many Baby Clothes Do You Actually Need in the First Year?
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