The subconscious mind is always picking up it’s surroundings, even when you’re asleep. The subconscious knows no boundaries. I have had many clients fall asleep during sessions, which tend to be very relaxing. I have found that many clients still see successful changes even if they fall asleep. All you would need to do is evaluate how you are responding after the session. Are you experiencing changes in the area of your goals, even if slightly? If so, then you should be fine. However, if you’re not sure, try changing positions when you listen to a file. Sit straight up with your legs on the floor, or some other position that you, personally, would not find too comfortable to where you drift into sleep.