There is no harm in falling asleep while listening to these meditations, in fact, if your body requires sleep, it is a good thing. However, there’s a good chance you will not be getting the extra benefits that are embedded within the meditations.
Sometimes, you may feel like you’ve fallen asleep when actually you have merely been in a very deep meditative trance. The best way to tell is if you wake up at the end of the meditation. If you do wake up at the end, this means your unconscious mind was listening to everything and you were most likely in such a deep trance that it felt like sleep.
If you find that you wake up sometime after the meditation has ended, there’s a good chance that you had fallen asleep.
If you find that you are falling asleep regularly when listening to these meditations, it may be good to change the time you are practising them or looking into other reasons that may be causing this. For instance, tiredness and exhaustion from lack of sleep, the positions you are meditating in, maybe you are run down and need to recuperate, etc.
If you have a schedule to keep, it is a good idea to set an alarm to wake you up after the meditation just in case you do fall asleep.