Room temperature, tog, and what to stop worrying about
A short guide to dressing a baby for sleep that does not require you to own a thermometer for every room.
Tog is a measure of thermal insulation, nothing more. A 2.5 tog sleep bag traps roughly two and a half times the warmth of a 1.0 tog one. It says nothing about the season, the fabric, or the brand, and comparing tog between two makers is one of the few baby comparisons that is genuinely like for like.
The confusion comes from packaging, which tends to be written in seasons. Seasons are a poor proxy. A bedroom with the heating on in February and a bedroom in an unheated attic in September can sit at the same temperature.
Go by the room, not the month
The only input that matters is how warm the room actually is while the baby is asleep in it, which is usually cooler than the room felt when you left it and warmer than you expect if the door has been shut.
A single cheap room thermometer, left in the room the baby sleeps in, removes the entire guessing game. It is the least interesting purchase on this list and the one that saves the most second-guessing.
How to check, without waking them
Hands and feet are a bad signal. A newborn has poor peripheral circulation and cold hands are normal at any temperature, which is why so many babies end up overdressed.
The back of the neck, or between the shoulder blades, tells you what you need. Warm and dry is right. Damp or hot means one layer too many. Cool there, rather than at the extremities, means one layer short.
Layers beat heavier bags
When a room sits below the range your bag is rated for, the useful move is to add a layer underneath rather than to size up in tog. A long-sleeved bodysuit under a 1.0 tog bag is easier to adjust than swapping between three bags, and it costs less.
The same logic runs the other way. In a hot room, the answer is fewer layers under a light bag, or a muslin swaddle instead of a bag, rather than a bag with nothing underneath it.
The one that is not about warmth
Fit matters more than tog. A sleep bag that is too large lets a baby slip down inside it, and that is a genuine risk rather than a comfort question. It is the reason we tell people to size down on sleep bags and size up on everything else.
Check the neck opening rather than the length: if you can pull the neck over the shoulders while the bag is done up, it is too big, whatever the label says.
What to stop worrying about
Whether the bag matches the cot. Whether you have the right tog for the exact week of the year. Whether the baby who kicks the blanket off is cold, because a sleep bag is the answer to that and it stays on.
The things worth being careful about are few, they are consistent across every reputable source, and they are listed at the bottom of this page rather than paraphrased in the middle of it.
