Box breathing is a great tool for athletes to have at their disposal. Two main ways to utilize it.
1. During competition - During a timeout, or a break in action, between plays, etc. Using 1-3 box breaths can help calm and slow down the body while delivering more oxygen to the body and enhancing carbon dioxide tolerance.
2. Before bed - Again this will aid relaxation and stimulate the parasympathetic nervous system to better prepare the body for sleep. Aim for 10 breaths of box breathing and you'll be well on your way to better sleep.
Nasal breathing is arguably the most powerful thing you can be doing every single day.
We are not meant to chronically breath through or mouths and it has disastrous long-term effects.
As you sit throughout the day and prepare for sleep (and during sleep) we NEED to be breathing through our nose. It filters, warms and moisturizes the air as it passes to our lungs (the mouth does not do this). It also increases nitric oxide (a vasodilator) which helps the passage of oxygen rich blood to our body. Nasal breathing also stimulates our parasympathetic nervous system (rest and digest) to allow us to recover and rest better (mouth breathing does the opposite).
Sleep disorders, apnea, snoring are all linked to mouth breathing which obviously disrupts our main recovery tool...sleep. Consider nasal strips or a nasal dilator, both I've used and they really do help with getting air through the nose during sleep.