Left Nostril Breathing is an extraordinary way
to activate our Parasympathetic Nervous System
The left nostril breathing is an anti-anxiety and relaxing technique.
Research studies have shown that conscious alternate breathing can dramatically alter both metabolism and the autonomic nervous system.
The left nostril breathing practice is a rhythmic type of breathing that turns on the parasympathetic nervous system (slows you down, calming, reflective, relaxing), and at the same time speeds your metabolism and gives you relaxed alertness because it balances the sympathetic nervous system as well.
How to do it?
With your mouth closed, just cover your right nostril and breathe in and out through your left nostril only. Start with one minute and build it up adding one minute a day, as you feel comfortable. You will experience an increase of calmness in your nervous system very soon. It would be great if you could practice this tool everyday consistently for faster results. 
Benefit
The left nostril breathing will calm down your anxiety. It will give you a sharper, clearer focus of the mind, and a deep, full relaxation.
It is said in the Yogic Traditions that if a person breathes through the left nostril for 31 minutes a day for 90 days (the time to install a new habit in your subconscious) he/ she will naturally change his/ her metabolism in favor of relaxation.
Research studies have shown that conscious left nostril breathing can dramatically alter both metabolism and the autonomic nervous system. Here is one study conducted by the National Institute of Health (NIH.)