What might the cause to of sudden breathlessness during sleeping?

Sleep disorder.... A number of problems could be going on. A night terror or bad dream could bring on acute anxiety, and then feeling of breathlessness. Sleep apnea -abnormal sleep state with prolonged pauses of breathing, then suddenly and loudly (sneeringly) starts again; there is repeated awakenings, and then somnolence during the day to point of falling asleep. See internist and/or pulmonologist.