Today is a solo episode because I was so blown away by this paper that I just had to do a whole podcast on it.
Here is the link to Heartmath, which has devices that can determine your best breathing rate for optimising your Heart-rate Variability.
Note that if you don't want to spend the money, you'll get the vast majority of the benefits of a 10 second breath cycle as either 5-5, or in for 4, out for 6.