My plan is to bring my down jacket that I bought from Decathlon for about £30 and a pair of warm leggings. That's what I did for the Trans Alba from what I'd learned from using a sleeping bag on Trans Atlantic Way.
When I woke up, all warm and cosy in my sleeping bag, I didn't fancy going out into the cold very much and didn't enjoy packing away y sleeping bag, sometimes while shivering. And it took a few miles to get warm.
But on Trans Alba, I used a down jacket and leggings. They are lighter and take up less space than my sleeping bag but are pretty much just as warm. Plus my arms and legs aren't restrained. Easier to put on a coat than crawl into a sleeping bag too. But best of all. When I wake up, I can stay warm! Just get on my bike wearing my coat and get going until I feel so warm that I want to take off the extra layers.
Just need somewhere sheltered if it rains and something comfortable to lay on.
Though, I'd still use a sleep control if that option is best and is available.
It's also very handy to have a warm coat anyway.