I have noticed the same, but I assume it is to do with the cold. As you probably know air expands with heat and the converse is true. Most nights on the hill get colder as they progress and the air in the bag gets looser!

Hi, and thanks for your feedback...all very useful. Much appreciated. See you in May!

It's all been said before...
X-lite or x-therm.
Either is good.
If I didn't have an exped down mattress I would buy an x-therm.
I'll be using the x-lite, it is fine for May, I have used one (neoair before) on my last 8 Challenges.