A light 0.5 TOG sack over a short-sleeve bodysuit.
For a baby up to a year, in a 77°F room: short-sleeve bodysuit and sleep sack (0.5 TOG).
At 77°F the room is on the warm side, so the sack is the lightest one: 0.5 TOG, over a short-sleeve bodysuit.
The makers' charts give 0.5 TOG for a 75–77°F room, and Baby Layers follows them.
Short sleeves under the sack keep the arms cool. If the neck feels damp, one layer less is the usual next step.
A 0.5 TOG sack for a room of 75–77°F.
77°F is warmer than the 61–68°F the Lullaby Trust recommends for a baby's room. Lighter layers help, and cooling the room a little, if you can, helps most.
At 75°F the answer stays the same.
At 79°F it changes to short-sleeve bodysuit, no sack.
Every night, whatever the room: Never a hat for sleep — a baby cools through the head. A sleep sack, never a loose blanket. Safer slightly cool than too warm: nothing makes the sack warmer than the chart. A fan can move the air — just not pointed at the baby.
0–3 months: short-sleeve bodysuit and sleep sack (0.5 TOG). 3–6 months: short-sleeve bodysuit and sleep sack (0.5 TOG). 6–12 months: short-sleeve bodysuit and sleep sack (0.5 TOG). 1–3 years: short-sleeve bodysuit. 0–3 months, swaddled: short-sleeve bodysuit and swaddle.
Runs warm: the app dresses a touch lighter, short-sleeve bodysuit, no sack.
Runs cool: asleep the app never goes warmer than the chart, so the answer stays the same. If the chest or neck feels cool, a layer under the sack helps.
Check the neck — warm but not sweaty means just right.