To sleep comfortably isn't really all that complicated, no matter if it gets cold inside. One el-cheapo but beefy sleeping bag, something that works as a sleeping pad and something that substitutes as a pillow. The last two can usually be improvised.

Instead of a sleeping bag you can of course improvise with blankets, but it is more hassle to make them work.

Forget about sleeping in bivy's inside. They're clammy beyond belief. And the next day will be miserable because you'll spend it in your office in clammy and moist clothes...

Bivy's have their place when you're sleeping OUTSIDE exposed to wind, rain, sleet and snow. Then you're all of a sudden much more motivated to tolerate a bit of dampness.