Blanket

A poncho liner or wool blanket is required for all participants.

These are handy not just as additional cozy layer when sleeping but also to pack along for LP/OP duty or for those cold night guard shifts.
When not needed for warmth, they also make great pillows. Many of us had them last year and were glad that we did.
You can find good old USGI scratchy wool blankets lots of places for generally less than $15.
Poncho liners are lighter and a little more comfy to sleep with (although many prefer the wool blankets) and cost about the same.