I spent a night in the woods (on purpose, not a real emergency) sleeping with an emergency blanket. It was not at all comfortable, but the blanket seriously worked. I would wake up whenever it moved to stop covering me because it would instantly become cold, then feel much warmer as soon as the blanket is back in place.

The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.