A four-degree window — 16 to 20°C — is narrower than most household heating systems naturally hold a room overnight, especially in older UK housing. There are two well-evidenced reasons it matters: overheating is one of the most consistently identified modifiable SIDS risk factors, and a warm room actively blocks the small temperature drop the body uses to fall asleep in the first place. A cold room is rarely dangerous, but it fragments sleep, particularly in the early hours when household heating tends to dip.
Healthbooq covers safer-sleep environment setup across seasons and housing types, with the same numbers as the Lullaby Trust and NHS guidance.
Why 16–20°C Specifically
Two distinct mechanisms make this range matter — they are independent and both supported by evidence.
1. Overheating and SIDS. Overheating is one of the most consistently replicated modifiable risk factors in case-control studies of sudden infant death (Lullaby Trust UK; AAP Task Force on Sudden Infant Death, 2022 policy statement). The presumed mechanism is impaired arousal: an overheated infant is less able to wake themselves from deep sleep when their physiology requires a wake. The same arousal-impairment is implicated in the prone-sleeping risk, suggesting these are related pathways rather than independent.
Risk rises particularly when overheating is combined with other risk factors — too many layers, head covered by bedding, prone sleep, parental smoking, sleeping somewhere other than a cot.
2. Sleep onset depends on a temperature drop. Falling asleep is preceded by a small core body temperature drop — about 0.3–0.5°C — driven by peripheral vasodilation (warm hands and feet shed heat). A warm room blunts the gradient and slows the drop. Sleep onset takes longer, sleep is lighter, and overnight wakings increase. This is the same mechanism that makes a warm bath such a good bedtime cue: the temperature rise during the bath, then the drop afterwards in a cool room, accelerates the body's natural sleep-onset signal.
A room in the 16–20°C range supports both: below the SIDS-associated overheating threshold, and cool enough for the pre-sleep temperature drop.
Measuring the Room — Where to Put the Thermometer
Household thermostats typically read at adult head height (around 1.5 m), where temperatures often run 2–3°C warmer than at cot height. A digital room thermometer placed near the cot — not on the cot itself, not next to a radiator — is much more accurate. £5–£10 buys one accurate enough; you don't need a smart device.
Rooms that look identical can run very different temperatures: a south-facing room in summer, a top-floor flat with poor insulation, a corner room with two external walls in winter. Measure the actual room before assuming.
Across Seasons
Summer. The priority is preventing the room from overheating in the warmest part of the night (typically 22:00–02:00 in flats; later if the room sits over a kitchen).
- Close curtains and blinds during the day in rooms that get direct sun
- Open windows in the cool evening (or earlier if temperatures permit)
- A fan circulating air helps; do not point it directly at the cot
- Move feeding chairs, bookcases, and laundry out of the room — anything blocking airflow
- A 0.5 tog sleeping bag with a vest only is usually appropriate from about 24°C; above 26°C, vest or nappy only with no sleeping bag
Winter. Central heating in many UK homes is programmed to dip overnight — a room at 19°C at 22:00 can drop to 14°C by 05:00. Two equally workable approaches:
- Programme the heating to hold a steadier overnight temperature in the cot room
- Or accept the dip and use a thicker (2.5 tog) sleeping bag with appropriate layers underneath
Draughts near the cot are worth checking explicitly: an old single-glazed window or an external wall can produce a localised cold spot that the room average doesn't reflect. Move the cot off external walls if possible.
Checking Comfort by Touch
The thermometer tells you the room. The dressing layer is what the baby is actually wearing. The verifying check is to place a hand on the baby's chest or the back of their neck once they are asleep:
- Warm and dry — dressing is right
- Hot or sweaty — too warm: remove a layer or lower the room temperature
- Cool on the chest — too cold: add a layer or check for draughts
Hands and feet are not a reliable comfort signal. Peripheral circulation in babies is less developed than in adults; cool extremities are normal even when the core temperature is fine. Adding layers in response to cold hands is one of the most common ways babies end up overheated.
Sleeping Bag Tog by Room Temperature
Tog ratings reflect insulation, not warmth as such. The right tog depends on the actual measured room temperature, not "is it cold outside" or seasonal habit. The general guidance from the Lullaby Trust:
| Room temperature | Sleeping bag tog | Layers underneath |
|—|—|—|
| Below 16°C | 2.5 tog | Long-sleeve vest + sleepsuit |
| 16–18°C | 2.5 tog | Vest + sleepsuit |
| 18–21°C | 1.0–2.5 tog | Vest + light sleepsuit; lighter as room warms |
| 21–24°C | 0.5–1.0 tog | Vest only or short-sleeve sleepsuit |
| Above 24°C | 0.5 tog or no bag | Vest only or nappy only |
The Lullaby Trust publishes a more detailed chart on their website worth checking against your specific sleeping bag and conditions.
Signs Your Baby Is Too Hot
- Sweating, damp hair, especially at the back of the neck
- Flushed cheeks (above background flush from sleep)
- Faster than usual breathing when calm and not unwell
- Restless sleep, frequent partial wakes, kicking covers off
- Skin hot to touch over the chest
If any of these are present: remove a layer or lower the tog. Do not try to use a wet flannel on the forehead — the goal is steady cooling, not a thermal shock.
When the Room Is Hard to Keep in Range
Some rooms — older flats, top-floor rooms in summer, north-facing rooms in winter — won't stay in the 16–20°C range without help. Two practical positions:
- Cold rooms (down to about 12°C): a 2.5 tog sleeping bag with appropriate clothing handles this safely. Verify with the chest/neck check.
- Warm rooms (above 25°C): vest only, no sleeping bag, fan circulating, blackout to keep the room cooler during the day, window open to cooler outside air whenever possible. Verify with the chest/neck check.
Across all situations, the touch check is the closing step. The thermometer and the tog chart get you most of the way; the chest or back-of-neck temperature confirms.
When to Get Advice
- A baby who is hot to touch and unwell (off feeds, listless, persistent cry, fewer wet nappies) — call NHS 111 or GP same day. A fever is not the same as an overheated room and needs separate assessment, especially under 3 months (any temperature ≥38°C in a baby under 3 months → NHS 111 / A&E).
- Persistent overnight sweating in a baby who is not in a warm room and is dressed appropriately — flag to GP or health visitor.
Key Takeaways
16–20°C is the Lullaby Trust / NHS / AAP recommended room temperature for infant sleep. The window is small for two real reasons: above 20°C is a recognised modifiable SIDS risk factor (impaired arousal from deep sleep), and a warm room blunts the small natural drop in core body temperature that the body uses to fall asleep. Below 16°C is rarely dangerous on its own — sleepwear handles it — but it disrupts sleep. The reliable comfort check is the chest or back of the neck (warm and dry, not sweaty); hands and feet run cool in babies even when the core is fine.
