We went from a few glorious fall weeks of 60 degree weather to 40 and raining overnight. Over a cold, cold night that made Lara wake up and moan "I'm cold, I'm so cold, Daddy!" from her room (which for some reason is much colder than the rest of our apartment). I know - I felt awful.
So I made her two pairs of new jammy pants out of a fleece baby blanket that I took apart into its two layers. I used this tutorial, which is spot-on, and the whole thing took about an hour - and that's including the time spent dealing with Jakey every five minutes. The next night she didn't wake up at all and in the morning I asked her if she slept well - "Yes, it was warm in my new toasty pants," said she.
I made myself a pair the next day out of a couple of Jakey's flannel receiving blankets!