Saw this recently:

Room temp 26 degrees plus - too hot - no covers
Room temp 24-26 degrees - hot - one sheet
Room temp 22-24 degrees - warm - one sheet, one blanket
Room temp 20-22 degrees - normal - one sheet, two blankets
Room temp 18-20 degrees - cool - (can't remember the recommendation here!)
Room temp below 18 degrees - cold - sleeping bag, sheet, 2 blankets.

Our bedroom is usually 18-20 degrees, DS has a sleeping bag at first then I cover him up with up to 2 sheets 2 blankets when I go to bed, depending on how cold the room is.

I sleep in PJs and under a 4 tog duvet and am sometimes a bit shivery, so I figure DS may be cold too. When it's warmer (underwear and sheets for me) he has a long-sleeved vest on and a sleeping bag, nothing else.