Hi, In winter, my 8 month old's room varies in temperature significantly from the time she goes down at night until she wakes in the morning. When she goes down, it's usually 21 degrees however by 4am, it has dropped to 15. The tog that is appropriate for 21 is way too thin for 15 degrees and visa versa. Keeping the room temp stable is not possible as I don't like keeling the split system on at night. If I dress her for 15 (3.5 tog, 1.0 onesie and wonder cool singlet) despite it being 20/21 degrees, what is the overheating risk? Is my only option to change her half way through the night?

It can be difficult to work out what may be the most appropriate clothing/bedding for baby when sleeping.
Red Nose recommends dressing baby according to the room temperature, rather than trying to modify the room temperature with heating or cooling.
Use light weight bedding & clothing, so that you can add or remove if baby wakes & feels too hot or cold.
You are not expected to get up & change baby through the night, or keep heating on overnight.
I wish I could give you some specific guidance for dressing your baby at night, but every baby is different.

Red Nose has the following general information about bedding/clothing amount & overheating.
Babies control their temperature predominantly through their head and face. This is why we recommend that you put baby to sleep on their back with head and face uncovered.
Red Nose does not recommend a specific room temperature for baby’s sleep. This is because there is no evidence to show that maintaining a specific room temperature prevents sudden infant death.

As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.
https://rednose.org.au/article/how-to-dress-baby-for-sleep

Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal).
Whether the baby has a cold or infection or another special need.
Consider how many layers that you as the baby’s carer are wearing comfortably.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep