The safe-sleep guidance most British parents grow up hearing — back to sleep, feet to foot, no loose blankets — exists because each line of it tracks a specific risk factor identified in the research that drove SIDS rates down by roughly 80% from the early 1990s onward. Overheating is one of those risk factors, and one of the most easily missed: babies overheat far more often from being overdressed in a normally warm home than from being put down in a too-hot room.
The aim of this piece is to give you the temperature numbers, the tog ratings, and the practical checks that mean you don't have to think about it once you've set the room up. The Healthbooq app covers infant sleep and safety through the early years.
Where the Overheating Risk Comes From
The link between overheating and SIDS isn't speculative — it came out of the same case-control work in Bristol, New Zealand, and Tasmania in the late 1980s and early 1990s that established the prone sleep position as the major risk factor. Peter Fleming's group at Bristol (the CESDI SUDI study, 1993–1996) found that excess thermal insulation roughly doubled the risk of SUDI, and that the effect was strongest when combined with other risks (room sharing absent, head covering, parental smoking).
The mechanism most likely involves arousal. Healthy babies wake briefly many times overnight in response to small physiological perturbations — a momentary apnoea, a head turn, a covered nose — and these micro-arousals are what protect them. Heat stress dampens that arousal response. If a baby in the second half of the first year (the peak SIDS window, 2–4 months) ends up face-into-mattress under a thick blanket in a hot room, the chain of events that should wake them is harder to trigger.
Two things follow from this. First, the risk is real but modifiable — the things to do are concrete. Second, "warm" feels reassuring to a parent and is not a good guide. Babies do not need to feel warm to the touch on their hands; their hands run cool because peripheral circulation in the first months is variable. Use the back of the neck or chest, where the body actually is.
The Numbers: Room Temperature
UK guidance — Lullaby Trust, NHS, NICE — converges on 16–20°C (61–68°F) as the target for a sleeping baby's room. This is cooler than most British central heating defaults, particularly in winter, and noticeably cooler than what a thinly dressed adult finds comfortable.
A few practical points:
- A room thermometer is worth £3. Guessing is the most common source of overheating. The cheap digital or LCD thermometers sold for nursery use are accurate enough; one in the room where the baby sleeps, ideally near the cot but not in the cot, is enough.
- The thermostat reading is not the cot reading. A cot under a south-facing window in summer, or next to a radiator, can be several degrees warmer than the room average.
- 20°C is the upper end of the comfort range, not the target. If you have a choice, 18°C with appropriate clothing is closer to ideal than 22°C with lighter clothing.
- In a heatwave, getting under 20°C overnight may not be possible. The fallback is to drop the tog of the sleep bag (or move to vest only), open windows on opposite sides for cross-ventilation in the early evening, draw blinds and curtains during the day to keep solar gain out, and check the back of the neck more often.
The Numbers: Tog Ratings on Sleep Bags
The reason Grobag-style baby sleep bags became the UK standard is that they make the maths trivial. Tog ratings give you a fixed insulation value, so once you know the room temperature you know what to put on:
| Room temperature | Sleep bag tog | What goes underneath |
|—|—|—|
| Above 24°C | 0.5 tog (or just a vest) | Short-sleeve vest, or nappy only |
| 21–23°C | 1.0 tog | Short-sleeve vest |
| 18–20°C | 2.5 tog | Long-sleeve vest, or vest + sleepsuit |
| 16–18°C | 2.5 tog | Long-sleeve vest + sleepsuit |
| Below 16°C | 2.5 tog + extra layer | Vest + sleepsuit + thin cardigan |
These are the figures most major UK manufacturers (Grobag/Tommee Tippee, The Little Green Sheep, Mori) publish and are consistent with Lullaby Trust guidance. If the bag is well-fitted at the neck and arms (so the baby can't slip down inside it), nothing else goes in the cot — no top blankets, no duvets, no quilts, no pillows.
What Goes In the Cot — and What Doesn't
The cot environment carries the temperature guidance:
- Firm flat mattress, fitted sheet, sleep bag — this is the whole list.
- No loose bedding. Blankets and duvets are not just suffocation risks; they're the most common route to overheating because there is no fixed insulation value and they migrate over the night.
- No cot bumpers, no pod, no pillow, no soft toys for under-12-months. Bumpers in particular have been associated with both overheating and asphyxia.
- No hat indoors. Babies lose a substantial portion of heat through the head; a hat indoors is one of the most common single causes of overheating, especially in winter when they were dressed for going out and not undressed on coming back in.
Swaddling deserves a separate mention. A correctly swaddled baby (arms in, hips loose, light cotton or muslin, no head covering) sleeps cooler than a duvet-covered one but warmer than the same baby in a sleep bag — drop the room temperature or the underneath layers slightly when swaddling. Stop swaddling at the first signs of rolling (typically 3–5 months), as a swaddled baby who rolls cannot reposition.
How to Tell If Your Baby Is Overheating
The standard check, taught by every UK health visitor: place your hand flat on the back of the neck or the centre of the chest, under any layers. It should feel warm but not damp or sweaty. Hands and feet are unreliable — they run cool even on a comfortable baby.
Other signs, from earlier to more concerning:
- Sweat on the hairline, neck, or upper back — the most reliable early sign.
- Damp hair, especially the patch where the head rests on the mattress.
- Flushed cheeks or chest that doesn't fade after a few minutes off any covering.
- Restlessness in sleep with kicking and head-shifting beyond the normal startle.
- Faster than usual breathing at rest.
- Lethargy or unusual difficulty rousing for a feed in a baby who is otherwise well.
If you find any of the first three, take a layer off (open the sleep bag, remove the sleepsuit, leave them in vest only) and check again in 10 minutes. If a baby is hot, drowsy, refusing feeds, or has sunken eyes / dry nappies / a high-pitched cry, treat that as possible heat illness and call NHS 111 or your GP — a baby with heatstroke is a 999 call.
The Common UK Scenarios That Catch Parents Out
In practice, the calls health visitors take about overheating tend to come from a small set of situations:
- Coming in from outside in winter still in pram suit and hat. The pram suit is rated for being outdoors at 4°C; transferred straight to a 21°C kitchen, the baby is now in roughly four times the insulation they need. Strip back to vest and sleepsuit on coming inside, even if you're going out again in twenty minutes.
- Car seat to cot transfer with extra blankets. Car seats should not be padded with bulky outerwear behind the harness (it disengages the harness in a crash); but the corollary is a blanket or footmuff over the harness, which then gets carried into the house and not removed at handover.
- Hot bath, then immediate cot. Core temperature stays elevated for 20–30 minutes after a hot bath. If the bath is part of the bedtime routine, allow that cooling-down period in lighter clothing before zipping into the sleep bag.
- Feverish baby buried in layers. The instinct is to wrap up a baby with a fever; the right move is the opposite — light layers, room on the cool side, paracetamol per dose schedule if they are uncomfortable. Wrapping up a feverish baby can drive the temperature higher.
- Co-sleeping under an adult duvet. If you are bedsharing (planned or accidentally), an adult duvet is the single biggest overheating risk because adult bedding has no tog rating and migrates over the head. The Lullaby Trust co-sleeping guidance covers this in detail; the short version is that bedsharing requires the baby in their own thin layer, on the parent's mattress level, with the duvet kept well away.
When Fans, Air Conditioning, and Open Windows Help
A fan in the room reduces SIDS risk independently of the temperature it creates — a US case-control study (De-Kun Li, 2008) found a roughly 70% reduction in SIDS risk when a fan was used in the sleeping room, hypothesised to be due to CO₂ dispersal around the cot. Aim it across the room, not directly at the baby; oscillating on a low setting is more effective than a strong direct breeze.
In the UK, domestic air conditioning is uncommon, so the realistic heatwave setup is: blackout blinds shut by mid-morning, windows opened on opposite sides of the home in the early evening once the outside temperature drops below the inside temperature (often around 7–8 pm), a fan running across the room, and lighter sleepwear. Cool flannel on the back of the neck before settling, room thermometer somewhere visible, check the chest at every wake.
When to Get Help
Call 999 / take to A&E:- Temperature 38°C or above in a baby under 3 months
- Temperature 39°C or above in a baby 3–6 months
- Drowsy, floppy, hard to rouse, weak high-pitched cry, sunken fontanelle, mottled or blue skin
- Repeated vomiting or refusing feeds in a hot baby
- Persistent damp/sweaty back of the neck despite layer reduction
- Fever in a baby with another concerning sign (rash, less than half normal feed intake, fewer wet nappies)
- Heat illness signs in older children: headache, dizziness, nausea after time outdoors
- A baby who repeatedly seems too hot in a normally cool room — it's worth talking through what they're sleeping in
- Repeated questions about tog and dressing in a context where every night feels different — bring a few photos of the cot setup to the next health visitor weighing-in
What Helps Long-Term
Set the system once. Room thermometer in the nursery, tog table on the inside of the wardrobe door, sleep bag matched to current room temperature, vest-and-sleepsuit (or just vest in summer) underneath, no hat indoors, no loose bedding in the cot. After about a week of doing the back-of-neck check on every settle, it becomes a habit and you stop having to think about temperature unless something changes.
The seasonal transitions are when overheating creeps in: the first week of unexpected April warmth, the first week the central heating is on in October, the day a heatwave hits and the sleep bag from yesterday is suddenly too warm. A note on your phone with the tog → temperature table is enough.
Key Takeaways
Overheating is one of the modifiable risk factors for sudden infant death syndrome (SIDS), alongside prone sleep position and parental smoking. The Lullaby Trust and NICE recommend a sleeping room at 16–20°C (61–68°F), a single thin layer plus a baby sleep bag rated for the room temperature, and the back of the neck or chest as the place to check warmth (not hands or feet). The most common overheating scenario in UK homes is not a hot bedroom — it's an overdressed baby in a 22°C room under a duvet or with a hat indoors. Tog ratings on baby sleep bags exist precisely so parents do not have to guess: 2.5 tog for 16–20°C rooms, 1.0 tog for 20–24°C, 0.5 tog for above 24°C.
