High levels of stress and anxiety often result in shallow breathing. Panic attacks are often caused by a lack of oxygen, so it is important to remember to take slow, deep breaths. This increases the supply of oxygen to your brain, stimulating the parasympathetic nervous system and promoting rational, calm thinking. Similarly, a stressed mind can often lead to tense muscles, even after the panic has subsided. This sense of uncomfortability increases stress, so be sure to do some stretching or deliberate muscle relaxing as and when space allows during the flight.