I often start my classes with a short Nadi Shodhana, alternate nostril breathing, practice. I am aware that if you have not practised it before it can seem quite awkward, and even if you HAVE practised it before, it can seem a bit out there and woo-woo.
I know because when I started practising Yoga, I was very sceptical about a lot of this stuff!
Why make that failed star trek gesture with the hand and do that weird nose thing anyway!? But it can have tangible benefits.
It turns out that almost all of us breathe primarily through one nostril for parts of the day, and then switch to breathing primarily through the other nostril at other times. The ancient yogis wrote about this and modern science has confirmed it. Research has confirmed that breathing through the right nostril is linked to the sympathetic nervous system(the fight or flight part) whereas left nostril breathing activates the parasympathetic nervous system (the relaxation one). This is pretty much exactly what’s described in ancient yogic texts, albeit using metaphoric language.
They refer to two nadis (hence the name of the practice). The left nadi, ida (pronounced ee dah) is the feminine, lunar, cooling side, the texts say, whereas the right nadi pingala (pronounced peen gah lah) is the masculine, solar, heating side.
Many of us live in a state of frequent, or constant, over-activation of the sympathetic nervous system . This is often the cause of anxiety, stress, and eventually increased blood pressure etc.
Others, such as some people with chronic fatigue syndrome, have too much parasympathetic tone. In either case, alternate nostril breathing can be useful.
Here is a link to a study showing how parasympathetic activity is significantly boosted by Nadi Shodana.