Baby Layers suggests a sleep sack, as the American Academy of Pediatrics advises a wearable blanket and no loose bedding. For a room of 16–20°C the Lullaby Trust also describes another way: a sleepsuit under a lightweight blanket, firmly tucked in. In the app it is a choice you can make, marked as sources differ, and the table shows it in those rooms.
What the app suggests for sleep, with the tucked-in blanket where it offers it, for a baby up to a year:
26–30°C: short-sleeve bodysuit. Warm room — a single light layer is enough, no sack needed. 24–25°C: short-sleeve bodysuit (0.5 TOG). A light 0.5 TOG sack over a short-sleeve bodysuit. 21–23°C: short-sleeve bodysuit (1.0 TOG). A 1.0 TOG sack over a bodysuit is a good start. 16–20°C: long-sleeve bodysuit, sleepsuit and lightweight blanket, tucked in. A sleepsuit under a lightweight blanket, firmly tucked in, in place of the sack. 12–15°C: long-sleeve bodysuit and sleepsuit (2.5 TOG). Cool room — add a sleepsuit under a 2.5 TOG sack.
With a blanket, as the Lullaby Trust describes: feet to the foot of the cot, so your baby cannot wriggle down under it, and the blanket firmly tucked in, no higher than the shoulders. Never a blanket over a sack, and one blanket doubled over counts as two.
From 26°C no sack is needed at all: a single short-sleeve bodysuit, and nothing on top. In a room between the two, or cooler than 16–20°C, the app suggests the sack, since it offers the blanket only for the room the Lullaby Trust gives it for.
A swaddled newborn wears the swaddle in the sack's place, until the first signs of rolling, around three months. Never a hat for sleep: a baby cools through the head.
Never a hat for sleep — a baby cools through the head.
Safer slightly cool than too warm: nothing makes the sack warmer than the chart.
Check the neck — warm but not sweaty means just right.