From Baby Centre:
What helps to keep him at the right temperature?
Choose a fitted sheet and layers of cotton blankets for your baby's cot, not a duvet. Keep plenty of extras on hand for changes, too.
Soft, one-piece, footed cotton sleepsuits help your baby stay warm all night by keeping him toasty from head to toe. When it's very cold in your baby's room, you could put a vest underneath his sleepsuit. Your baby does not need to wear a hat indoors or while he's asleep.
Feel your baby's tummy to see if he is getting too hot or too cold and adjust his bedding accordingly. If your baby is too hot, remove one or more blankets as needed. If your baby is cold, add a layer. Other signs that your baby may be too hot include sweating, damp hair, and heat rash.
Don't feel your baby's hands or feet to work out if he is warm enough; it is normal for them to feel cold. However, if they look blotchy or blue, you could add mittens, socks, or booties.
Remember, if your baby has a fever; he'll need less, not more, bedding.
Many parents use a baby sleeping bag that has a zip or poppers so your baby can't kick it off. This means that he cannot bury himself under the covers and he is less likely to wake up because he's feeling chilly. As sleeping bags are also sleeveless, your baby can still move his arms around while the rest of his body stays covered. Sleeping bags come in different tog ratings. For most of the year, a 2.5 tog sleeping bag is suitable.
If your baby's room is colder than 16 degrees C, you could consider using a 3.5 tog sleeping bag.
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