Here are the problems with being told to count to 4, 5, 6 while breathing in and doing the same breathing out by various sources (newsletter from our medical provider, yoga books, stress management advice, etc.):
Average rate of breathing: I have been watching the number of breaths per minute, called eupnea, in the age range of 14 to 87 years. It is about 15 breaths per minute. It is in the range of the figures cited by a Bihar School of Yoga book on prana. Going online, Wikipedia confirms that the observation falls roughly in the middle of the range. So assuming 15 breaths per minute, each breath is 4 seconds or a count of 4–2 in and 2 out. It is also counted by counting the number of times the chest rises in one minute.
Stretching the breath: So if people are asked to double or triple the duration of their breath, they are likely to strain their lungs, have discomfort in the chest, and feel light-headed. Longer breath is more calming but there is a method to it. Before beginning a breath control exercise, observe the length of your own natural breath.
Those of us who for years have trained ourselves to breathe from the belly may not have a significant rise in the chest and that may affect the counting based on chest rises.
Chest breathing versus belly breathing: Most people are chest breathers. Try breathing through the chest and lengthening the breath (I did for this blog post) and see what happens. The breath creates tightness and discomfort in the chest. After a few of these breaths, people may experience gasping and shortness of breath. A short breath seems to feel fine in the chest but not a long one. The chest fills up quickly and it is a more stressful breath. It is good when you need quick bursts of oxygen in times of stress. However, if you learn to breathe from the belly (also called abdominal breath) it is much easier to lengthen and deepen the breath. There is no tightness, no discomfort as long as you don’t overdo it. The back must be straight and the chest very slightly raised, otherwise the breath is constricted, strangled, and short. This breath is relaxed and soothing.
Gradual lengthening: The breath cannot be doubled or tripled at once. The lungs cannot be stretched that much without strain. It must be done gradually, slowly. This is a step-by-step, systematic process: relax the body and mind, become aware of the body, and become familiar with the breath. Then breathe with the whole body flushing out the respiratory system and gently stretching it with comfort. This brings awareness of the breath in all parts of the respiratory system. Then teach the body how to breathe from the belly and retrain it. The body is then ready to pace the breath to a comfortable count and adding one second at a time till the breath settles to the new pace–just like in exercise, repetitions and sets are gradually added. Adding sound is an effective way to stretch the breath.