“Taken together, our results suggest that slow-paced breathing performed before sleeping may enhance restorative processes at the cardiovascular level during sleep.”
This excellent study (see the Science 411 for more) found that 15 min of slow breathing (6 breaths/min) before sleep for 30 days led to:
Significantly better subjective sleep quality
Significantly higher nighttime vagal tone (via HF-HRV)
Higher (but not significant) morning vagal tone
That’s impressive for just 15 minutes a night 👏
P.S. To be honest, I’ve never been able to commit 15 minutes before bed to breathing. But I always do at least 5-10, and I find it more than enough to fall asleep faster.