This is definitely not the answer you are looking for, at least if you are like most people on here...but it a rather interesting different way to think about the situation at hand and to have a whole new kind of experience on a bike trip.

Sleep during the day and ride during the night. Experience where you are going to ride under a totally different set of circumstances. Now you sleep when its warm and ride when its cold. Your body warmth, aka body pedaling the bike will keep you warm over night while the sleeping bag will keep you warm during the day while you sleep.

Could provide a nice chance, depending on where you are going, to see the northern lights while riding the bike...have you ever did that before?

On the normal side of the equation...the one thing I have learnt many times over is anything placed directly on top of the body, no separation to the outside air will provide insulation. Placing a rainfly directly on top me on a 60 degree night, no other protection, no tent, no sleeping bag, keeps me warm. I would be cold without the simple piece of nylon tossed on top of my body. You just need something to help trap the body heat in and it needs to be placed directly against the body/clothes you have on, no air gap between it and the body/clothing.