i would say its all about the "inner you" (sorry, im not going guru) and probably not much to blame on your lifestyle.
before an interview chill out and have a clear head. when being spoken to, keep this calmness and just concentrate on what the person is asking. take a breath, run through it quickly in you mind and clearly and easily reply.
remember - when under stress, what seems a long time to you is actually very short to others.
being calm and collected will show you are responsible for the job.