Breathing

Sometimes when I'm concentrating on something (usually writing or attempting to make numbers add up properly, since there's very little else I bother to concentrate on) I start to get a bit dizzy. Then I realise that I'm unwittingly holding my breath. I sometimes wonder if this unconscious respirational suspension could ever lead to actual unconsciousness, or if I will always become aware of it in time to remind myself to start working those lungs again. Either way, isn't breathing supposed to be an automatic thing? How can one just 'forget' to breathe?