in my experience, when i tried to do work and would fall asleep, it was my brain putting the brakes on; NOT READY! therapy and alter work reveals layers. the mind will only reveal what it is ready to reveal (or reveal what you can handle). It might be saying TIME OUT!

I also did automatic writing until the words popped in my head before I could get them to paper.

Remember your mind is also like a muscle - it too needs to recoup every now and then.