Gave this a try a few times over the last 24 hrs. Interesting way to control thought process and definitely takes a little more focus than just yoga style breathing.
I was gifted a Fitbit over the holidays and have been using it, essentially, to make myself get up and do household chores once an hour, 9-6, and to monitor my sleep schedule. It's led me to do more cardio and dramatically shift my sleep, and I noticed that I've started to wake up while deep breathing. So I guess there is some kind of general activity level thing going on that governs how you breathe, too.
Shameless self-promotion:
For those who are curious about monitoring breathing patterns: https://www.aidlab.com (a chest strap with a respiration sensor)