If you do nothing else, try alternate nostril breathing, called both nadi shodhana or anuloma viloma. Both names are in Sanskrit. There are thousands of nadis, energetic channels or meridians, that run throughout the body. Shodana is the process of purification, in this instance through breathing in and out both nostrils to cleanse the meridians. The words anuloma and viloma can be interpreted as regular and irregular, as there is a retention of breath in this practice.
Many breathing exercises have contraindications. But alternate nostril breathing can be appropriate for all. I deem it just as important as the child's pose or savasana (final relaxation). My Sivananda Yoga lineage instructs us to practice anuloma viloma before the physical asanas, every day.
In the first chapter of my book, "From the Boxing Ring to the Ashram," I suggest this pranayam technique as a one-minute time-out. It is the best recommendation I have for giving your parasympathetic system a quick jolt that yields a great sense of calm. I find this breathing exercise extremely relaxing. But there are many other benefits. It clears the sinuses and balances the left and right meridians, and hopefully the hemispheres in the brain.
There are different ways to practice nadi shodhana or anuloma viloma but all focus on inhaling and exhaling, slowly, through one nostril at a time.
The simplest way is to pinch your right nostril shut with your right thumb and inhale fully through the left nostril. Next, close the left nostril with the right pinky and ring finger and exhale through the right side. The index and middle finger either stay tucked into the palm or reach for the third eye, as in the image here. For the next round, begin by inhaling through the right, and exhale through the left. The inhale and exhale can be equal count. However, to rev up the parasympathetic system, inhale to a count of four, hold the breath for a count of six, and exhale to a count of eight.
Whichever version, try ten rounds.