yes indeed, this is it.
I've spent a while working on the 'sleeping system', about 2 years trying out various ways of staying dry, lightweight and comfortable (within reason). I'm now carrying a tent outer (Hilleberg Akto, urgghh, but another story), a 100% waterproof bivvy bag (Snugpak SF, about the size of a bag of sugar), and a sleeping bag (cheap Decathlon 7-degrees plus). Tent under the bars, bivvy bag in the bar-bag, sleeping bag in the tail pack. Mostly I'll bivvy, but on the wet nights (or if I end up in a campsite) I'll use the tent with the bivvy bag inside. Essentially the bivvy bag replaces the tent inner, and is more versatile as well as being smaller/lighter. I also carry a half-size 'sock like' mozzie net which slips over the top half of the sleeping bag. I try to do three nights wild-camping and one night in a hotel/hostel.
Under the saddle is a tool pack, plus a rear light attached. NBT2, chain links, chain tool, spoke wrench, punture repair kit, inner tube, multi-tool.
In the rear pack, along with the sleeping bag, are non-cycling shorts, thermal top, 2 x socks.
In the bar-bag, along with the bivvy-bag, are passport and credit cards, 'purse' for cash, 5" smartphone for GPS logging and digital needs, mini-tripod, head torch which doubles as bike front light, pen and notebook, knife, mozzie-net, and toothbrush/paste. A bunch of paper maps in the map-case. Also some space in the bar-bag for daily food (additional food storage in rear pocket of cycling jersey).
@jags, well.. Last trip to Norway, through the night, I had to get up every couple of hours and walk about to get feeling back in my feet. It was quite a bit below zero, and I was in the bivvy bag with a zero-plus sleeping bag. I wasn't sleeping because of the pain shooting through the feet, so no need for an alarm clock
An extreme example possibly, but 'going light' is probably just an exercise in pushing the boundaries of what you are able to tolerate.
The big draw, for me, is climbing mountains at 'normal' pace, descending fast without under/over steering because of luggage, sitting drinking a coffee and looking at how little I need to survive/thrive on the mountain roads after riding thousands of kms to get there. But above all, maybe, the sheer distance a bicycle will allow you to cover, 100km per day, 200km, 300km? It's a bit like being able to do an Audax ride, taking in the Stelvio pass. And then the following day another one taking in the St Bernard. On a bike that feels responsive, that you can sprint with, that's basically the bike you wheel out your front door for a day-ride.
It's also quite possible that I am simply not good at remembering stuff, and taking so few things means I am less likely to leave stuff behind
EDIT: ooh, and a lock wrapped around the bar-bag mount.
cheers,
Doug