What is well supported
Nasal breathing helps condition inhaled air. Research also indicates that the nose contributes substantially to nitric oxide in the airways. This does not make nasal breathing a cure, nor is it always appropriate during intense exercise or congestion.
A 7-day experiment
During easy walking or quiet work, breathe through the nose for five to ten minutes without forcing it. Record effort, mouth dryness and ability to speak. Stop if you feel air hunger, dizziness or pain.
What to measure
Compare heart rate, perceived effort and sleep quality, while changing only one variable. Treat the result as personal observation, not diagnosis.
Sources and further reading
Important: Practical suggestions are for education and self-observation and do not replace medical care.