Meditation and Breath (Pranayam)

Many meditations also use the breath, perhaps simply bringing your attentions to the flow of your breath, or by consciously using specific patterns of breath, such as regulating the ratio of inhale to exhale, or breaking the breath into segments, or indirectly through the use of mantra. Since breath is correlated with your moods and energy level, altering the depth, rate, and pattern of breath can change them as well.

Be sure to follow the instructions for using the breath in a meditation carefully, and check with an instructor if you have questions. Begin with short times and gradually increase them as you get used to the changes they cause. If you feel dizzy, stop and make sure you are using the proper technique. Breath meditations create a lot of change and it is important to feel comfortable and balanced with these changes

Check to make sure that you breathe with a complete breath pattern (please see the section on pranayam for a complete description of the long deep breath). About 30% of people do not breathe correctly, but it is easy to change, and will have a profound impact on your metabolism, vitality and moods. Unless the meditation specifies otherwise, breathing is always through the nose.
Source: AskMeditation

...................................................