I believe in stopping everything to participate in breathing strategies.
Every morning my 4th grade class would breathe for a minute. I introduced one at a time and once they were all introduced, the student of the day could choose which one they wanted to do. I always pose it as an invitation in case anyone does not want to do it. They are to remain quiet and respectful, but some choose not to participate.
Most of these breathing exercises can be found on GoNoodle! If they are not there, I recommend searching for them on YouTube. As always, preview videos before using them with your class.