The 4-7-8 breathing is popularized by Dr. Andrew Weil and is an effective to reduce stress, tension and anxiety, also through the technique blood pressure can be reduced and it can help with problems falling asleep.
Exercise Instructions:
First exhale through the mouth, then begin the 1st cycle: inhale through the nose for 4 seconds, hold the breath for 7 seconds, exhale through the mouth for 8 seconds. This cycle is then repeated 3 more times, or later you can also extend the practice and perform a total of 8 cycles.
If you find it difficult to hold your breath for so long, the technique can be accelerated, the rhythm is more important than the absolute duration, so instead of 4-7-8 seconds of breathing, it is perfectly OK to do 2-3.5-4 seconds, that is, twice as fast breathing.
You might not feel much of an effect at first, but once this becomes a regular practice this will have a pleasant calming effect very fast. Also it is an exercise that is extremely easy to implement because of its short duration.