I have some problems like these too. Yesterday at night, I went and sat on a couch. Then stretched and relaxed, and then concentrated in my breath, and cleared my mind. Soon after this, I started to feel like goosebumps all over my body, they came and went. And also heard a buzzing sound. Then I entered I think in a very light phase of hypnagogenic (however it spells) images, and when I notticed I was "sleeping" I could my heart race. And then it just faded away. I couldn't do anything more, so I stopped trying. Any suggestions about this? What can be done in this case, and also, how do you know you're ready to exit?