The Mind/Body Institute provides the following FAQ:

• Can the relaxation response "prevent" stress?
  Regular elicitation of the relaxation response can build a resiliency to stress. However, stress will continue to confront us.


• Do you recommend listening to tapes or just repeating a word or phrase to elicit the Relaxation Response?
  A person should use the mode that is most comfortable. Choose a technique that works for you.


• What if I fall asleep during the Relaxation Response?
  If you fall asleep while eliciting the relaxation response, it is not harmful, but you should continue your practice for 5-10 minutes after you wake to receive its benefits. If you fall asleep after, be sure to choose a sitting posture.
  (The Relaxation Response helps you to unwind if you have trouble falling asleep).


• What if I do not have twenty minutes a day to spare for the Relaxation Response?
  Any time that you have to perform the Relaxation Response is beneficial to you. A few minutes practice is better than none.