I've got a bunch of synthetic bags with ratings down to 0 F. Of those, I've got a couple that might be comfortable down to freezing. I've got an old REI down bag that I bought in the seventies that I have yet to have a cold night in. It's important not to have a bag that is too large, since you can waste heat on the empty space. I prefer a barrel shaped or tapered bag over a mummy bag and currently use a Climashield quilt down to freezing. Another important consideration is your sleep suit. Down to freezing, I wear an army surplus LWCWUS top and bottom with Merino wool socks and a Merino wool balaclava Buff. My sleep socks would not normally be worn during the day to keep them dry and un-matted.