I can't go into a lot of detail, but we just finished that unit a few weeks ago - the reticular activating system is basically "wakefulness drive" (not the exact definition, I know) as far as I can tell. So when the reticular activating system is not stimulated (ie during sleep in darkness, etc) ventilation does tend to slow. When you wake up and it's light, you're upright, etc breathing tends to speed up...