Sleep Sack
A wearable blanket designed to keep infants warm during sleep without the suffocation risk of loose bedding.
| Safe from | Birth (or when swaddling ends, typically 8–16 weeks) |
| Use until | 2–3 years (or when child climbs out) |
| Purpose | Temperature regulation without loose blankets |
| Material types | cotton, fleece, merino wool, muslin |
Safety Function
Sleep sacks eliminate loose blankets in the cot—the leading soft bedding hazard for infants. Unlike traditional blankets, which can cover a baby's face and obstruct breathing, sleep sacks remain secured at the shoulders whilst allowing leg movement. The NHS and American Academy of Pediatrics identify loose bedding as a SIDS risk factor; sleep sacks address this whilst maintaining warmth.
Key safety features:
- Armholes fitted (not loose)
- Neck opening snug but not tight
- Bottom remains open for nappy changes
- No hood attachments
Types and TOG Ratings
Sleep sacks use the TOG (Thermal Overall Grade) system to indicate warmth level:
| TOG Rating | Room Temperature | Season |
|---|---|---|
| 0.5 | 24–27°C | Summer, very warm rooms |
| 1.0 | 21–23°C | Spring/autumn, moderate heating |
| 2.5 | 16–20°C | Winter, cooler rooms |
| 3.5 | Below 16°C | Very cold rooms (rare in heated homes) |
Material considerations:
- Cotton: Breathable, year-round
- Fleece: Warmest option (2.5–3.5 TOG)
- Merino wool: Temperature-regulating, expensive
- Muslin: Lightweight, ideal for warm climates
Sizing and Fit
Sleep sacks are sized by age/weight, not clothing size:
| Size | Age Range | Weight |
|---|---|---|
| Newborn | 0–3 months | 2.7–5.4 kg |
| Small | 3–6 months | 5.4–7.7 kg |
| Medium | 6–18 months | 7.7–10.9 kg |
| Large | 18–36 months | 10.9–13.6 kg |
Proper fit indicators:
- Armholes allow one adult finger of space
- Length extends 10 cm past feet
- Neck opening doesn't slip over chin when arms raised
Transitioning to Sleep Sacks
From swaddling (8–16 weeks):
- Week 1 — Use transitional sleep sack with arm wings (allows gradual arm freedom)
- Week 2 — Switch to standard sleep sack with both arms out
- Monitor for increased night waking (temporary as baby adjusts)
Temperature check: Feel baby's chest or back of neck—should be warm but not sweaty. Cold hands/feet are normal and don't indicate the baby is cold.
Common Issues
Baby removes sleep sack: Once children can unzip or wriggle out (typically 18–24 months), switch to fitted sleepwear or consider a backwards-zip design.
overheating concerns: Dress baby in one layer less than you'd wear to bed. A vest and nappy under a 2.5 TOG sack suffices in 18°C rooms.
hip dysplasia: Choose sacks with adequate room at the bottom for leg movement. Hip-healthy designs allow the "frog position" (legs bent and apart).
When to Stop Using
Discontinue sleep sacks when:
- Child consistently climbs out of cot (safety risk)
- Child requests to stop (typically 2–3 years)
- Transitioning to bed with duvet (usually 18–36 months)
Some families continue using sleep sacks in toddler beds as a sleep association cue.
Consult a health visitor if:
- Baby shows signs of overheating (flushed face, rapid breathing, heat rash)
- Sleep sack causes skin irritation or restricts movement
- Baby's temperature regulation seems abnormal
See also: Swaddling, Safe Sleep Environment, Room Temperature, SIDS Prevention, Cot Safety