I've always been alright with the synthetics, and I slept in the coldest possible weather, prolly was -10 deg C, with a inner bag (homemade), synthetic Swiss army bag with outer bag, with South African poncho draped over the lot, in Sutherland, a semi arid region in South Africa. Snug as a bug.... These days we have even better - the cheap fleece blankets, a few of those and you should be fine. And I even got one better, I got me a swag, and now I'm allright for the -10's again.
Layering layering layering