The target is a room somewhere around 16 to 20 degrees, a baby dressed for that temperature and nothing more, and a quick chest check before you go to bed yourself. That is the whole system. The rest of this guide is the detail, including the two nights a year the system gets tested: the heatwave and the cold snap.
As with everything sleep-safety, the Lullaby Trust and the NHS are the authorities here, and where anything below seems to disagree with them, they win.
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How to tell if your baby is too hot or cold
Not by their hands and feet. Baby hands and feet run cool as standard, and generations of babies have been overdressed on the evidence of chilly fingers. Check the chest or the back of the neck instead: it should feel warm, not hot or sweaty, and not cold.
If the chest feels hot or clammy, or you find damp hair, flushed cheeks or fast breathing, take a layer off. Being a little cool is easier for a baby to handle than being too hot, and overheating is the one to actively guard against; it is linked to a higher risk of sudden infant death, which is why the whole subject earns a guide.
The room
Around 16 to 20 degrees is the guidance, and a simple room thermometer takes the guesswork out for a few pounds. Do not chase perfection: real bedrooms in real houses drift, seasons happen, and a degree either side is not an emergency. The temperature is there to inform what your baby wears, which is the part you actually control.
What to dress them in, by temperature
The system is layers plus a baby sleeping bag, because a sleeping bag cannot be kicked off at 3am and cannot ride up over a face. Tog is the warmth rating: the higher the number, the warmer the bag. As a working guide for a room at each temperature:
24 degrees and above: a sleeveless bodysuit with a 0.5 tog bag, or in real heat just a nappy under the 0.5 tog.
21 to 23 degrees: a short-sleeved bodysuit with a 1.0 tog bag.
18 to 20 degrees: a sleepsuit with a 1.0 to 2.5 tog bag depending on which end of the range your room sits.
16 to 17 degrees: a sleepsuit with a 2.5 tog bag.
Below 16 degrees: a vest under the sleepsuit with the 2.5 tog bag, and a look at whether the room can be brought up a little.
One honest caveat: sleeping bag brands publish their own charts and they do not all agree, so treat the chart that came with your bag as the tiebreak for that bag. The principles do not change: dress for the room, check the chest, adjust.
Heatwave nights
Above about 24 degrees, strip the system back: nappy plus a 0.5 tog bag, or just a nappy and, if you use anything at all, a single well-tucked sheet. Cool the room before bedtime rather than during the night: curtains or blinds closed through the hottest part of the day, windows open when the air outside is cooler than inside. A fan is fine to circulate air, aimed across the room rather than at the baby. A slightly cooler than usual bath before bed helps, cotton beats synthetics, and remember that a hot baby cannot kick off a sleeping bag, so err lighter.
Cold snaps
Add layers to the baby, not hazards to the cot. A vest under the sleepsuit and a warmer tog is the whole move. No duvets, no extra loose blankets piled on, and never a hot water bottle or electric blanket in or near a baby's sleep space. If you use blankets rather than a bag, firmly tucked and no higher than the shoulders, as covered in our safe sleep guide.
No hats indoors
Babies regulate heat through their heads, so a hat worn for sleep traps exactly the heat their body is trying to shed. Hats are for outside. Indoors, for sleep, bare head, every time, whatever the season.
When to talk to a professional
If your baby feels hot and seems unwell, that is fever territory rather than clothing territory: check their temperature properly and follow NHS fever guidance or call 111. And if your baby was premature or small at birth, ask your midwife or health visitor about temperature and dressing specifically, since the standard charts assume a full-term baby.
Quick answers
What temperature should my baby's room be?
Around 16 to 20 degrees. A room thermometer helps, and a degree either side is fine; adjust their layers rather than chasing the perfect number.
How do I know if my baby is too hot or too cold?
Feel their chest or the back of their neck, not their hands and feet, which run cold naturally. Warm is right. Hot, sweaty or clammy means remove a layer; cold means add one.
What should my baby wear to sleep in a heatwave?
At 24 degrees and up, a nappy with a 0.5 tog sleeping bag, or just a nappy in real heat. Cool the room during the day with curtains closed, and use a fan to move air across the room, not at the baby.
Should my baby wear a hat to sleep?
No. Never indoors. Babies release heat through their heads and a sleep hat risks overheating.
Dressed right, and someone to ask anyway
Charts are fine until the radiator breaks or the heatwave hits, and those moments have a habit of arriving at night. SlumberPlan builds a routine around your baby and stays on WhatsApp around the clock, so "it's 26 degrees in her room, what do I put her in?" gets an answer straight away, at the exact moment you need it. It is AI. That is why it is awake when the boiler is not. Both parents share one plan and one line, there is a 30-day money-back guarantee, and you can cancel anytime. Start at slumberplan.com.
SlumberPlan is currently available to families in the UK. We're not taking US or Canadian signups yet.