you generally are not advised to use blankets/sheets etc at all with a sleeping bag.

DS went in to a sleeping bag at 5 months. What to wear underneath depends on the warmth rating of the sleeping bag.

At the moment his room is usually around 16-18 at night when he goes to bed, but would drop to 14-16 overnight I suppose.

he wears a singlet, long sleeved bodysuit (no legs) and thinish/light flannel jammie pants, plus a 2.5 tog grobag. and some nights thats too warm for him so we take out the singlet.

Its getting too warm for his 2.5 tog bags though, we will be swapping to the 1 tog soon.