Which sleeping bag is suitable for which room temperature? The guide
De verwarming ging om tien uur uit. Het is nu half drie 's nachts en de slaapkamer is vier graden afgekoeld. Je baby ligt er nog precies zo bij als toen je hem neerlegde.
The heating went off at ten. It’s now half past two in the morning and the bedroom has cooled by four degrees. Your baby is still lying there exactly as you left them.
That’s the tricky thing about choosing sleep sacks: you make one choice for a night that changes.
Start with the room, not the sleep sack
Most parents start with the product. They search for “which sleep sack for winter” and end up with a number.
Turn it around. Measure your bedroom first — and not just once, but over several evenings at different times. Place a simple thermometer at the height of the cot, not on the windowsill or next to the radiator.
What you usually see:
- Around bedtime: the highest temperature of the night
- Between 3 and 6 a.m.: the coldest moment
- The difference between the two: often 3 to 5 degrees
Choose for the coldest moment, not the warmest. You can remove a layer from a baby who is too warm. A baby who cools down at four in the morning wakes up.
In the Netherlands, the recommended bedroom temperature for babies is around 16 to 18 degrees. Warmer is not better — a room that is too warm is one of the things child health clinics specifically look out for.
The guide
Room around 16–18°C — cool nights, typical of most Dutch winters
This is the scenario most parents face in winter. The heating is off at night and it’s freezing outside.
Merino wool sleep sack + long-sleeved bodysuit + optionally a thin wool sleepsuit underneath.
With a synthetic or cotton sleep sack, you’re looking at approximately 2.5 TOG here. Wool works differently: the fibre insulates by trapping air and continues to do so when the room is damp. That’s the difference you notice on a cold, clammy February night.
No loose blanket on top. This isn’t a matter of taste but of safety — don’t combine loose blankets and sleep sacks.
Room around 19–21°C — the transitional season
Spring and autumn. The trickiest category, because outdoor temperatures can swing wildly.
Merino wool sleep sack + long-sleeved bodysuit.
This is exactly where the TOG system leaves parents struggling: your summer sleep sack is too thin, your winter sleep sack is too thick, and you do not have an in-between option. With wool, you do not have to make that choice — the material adapts to the room.
Room around 22–24 °C — summer, or a well-insulated home
Merino wool sleep sack + short-sleeved bodysuit, or just a sleeveless bodysuit.
This is where parents often hesitate: a wool sleep sack in summer? Yes. Wool wicks moisture away from the skin and releases heat as temperatures rise. It is the same reason hikers wear merino wool shirts instead of cotton in summer: cotton gets wet and stays wet, while wool does not.
Room above 25 °C — tropical nights
Merino wool sleep sack + diaper only. Or, in extreme heat, just a short-sleeved bodysuit without a sleep sack.
Above 25 degrees, it is no longer about keeping warm, but about staying cool and releasing moisture. Check your baby more often. Ensure air circulation, close the curtains during the day, and never use a fan pointed directly at the crib.
Size is not a detail
A sleep sack in the wrong size is not merely uncomfortable — it is a safety issue.
The neck opening should be designed so that your baby cannot slip down into it. Test it: lift the closed sleep sack by the shoulders and check whether there is enough room for a little head to slip through.
The armholes should not be too loose, especially once your baby starts rolling over.
The length can be generous — little feet need room to move — but not so long that your baby can slide all the way down.
Never buy a larger size for your baby to grow into. Sleep sacks do not work like sweaters. Our sleep sacks are available in 0–6 months (60 cm), 6–12 months (75 cm), and 1–2 years (90 cm).
What I always tell parents
After 28 years in neonatology, I came to believe in one simple habit more than in any chart.
Feel your baby’s neck each evening, between the shoulder blades, just before you go to bed yourself.
Pleasantly warm: good. Clammy: remove a layer. Cool: add a layer.
It takes five seconds. And it is the only measurement that accounts for your baby, in your room, on this specific evening — something no table comes close to doing.
Hands and feet tell you nothing. They are almost always cooler than the rest of the body in babies. That is simply how circulation works in the first few months.
Frequently asked questions
What is the best way to measure the room temperature?
With a simple thermometer at the height of the cot, away from the radiator, window and door. Measure for a few evenings in a row at different times, so you know how much the room cools down at night.
Can I put a blanket over the sleep sack?
No. You should not combine a sleep sack with a loose blanket. The sleep sack is designed to make loose blankets unnecessary. If you think your baby is too cold, add an extra layer underneath the sleep sack.
Why don’t you recommend separate summer and winter sleep sacks?
Because merino wool adapts to the conditions. The fibre insulates when it gets cooler and wicks away moisture and heat when it gets warmer. Adjust the layers underneath, not the sleep sack itself.
My baby’s hands are sweaty in the sleep sack — is that a problem?
Hands and feet say little about body temperature. Check the neck. If it feels clammy, one fewer layer is enough. If it feels pleasantly warm, your baby is comfortable.
When can my baby sleep in a sleep sack?
From birth, provided the size is right. For all questions about safe sleep — sleeping position, mattress, and what should and should not be in the bed — always follow the current guidelines from your child health clinic or midwife.