It's not gonna get up to 45 until 11am, and we start a 6am. (It's a 300K, so an all-day affair.) I'm gonna err on the side of being too warm later in the day. I can deal with open flapping zippers, but I can't deal with being cold all morning. Thanks for the tips and advice. Good idea on carrying spare chemical warmers. If I'm out later than I planned, the temperatures will be dropping again, and if I DID overheat during the day, that sweat won't help keep me warm at night!