This exact same topic came up a couple of months ago.
Space blankets and bivys are designed for short-term emergency use. If this was a good idea, companies would be making and selling them for long term use. There are reasons they don't.
If you want to find out firsthand how this would perform, make one using a couple of blankets and duct tape and see for yourself.
There are many threads here on sleeping bag design and physiology. I suggest you read back through them for a better understanding of how a bag works and what your body is doing while in one.
Don't get me started, you know how I get.