Different brain waves, and binaural beats.
I'm posting this here because I don't have concrete evidence that they cause LDs but I seriously think it's what has made me have so many LDs. I've another post on here where I talk about this a little bit but about a year ago I downloaded and listened to a binaural beat that was an hour long one night before and during the time I was sleeping and since then I've had probably over 100+ LDs, before I ever listened to one my entire life I had maybe 3-4 if even that.
I'm on the DVs chat right now and I've introduced some people to binaural beats by talking about them, if you don't know what they are I'll post some links so you can better understand them. All they are are basically the waves (alpha, beta, delta, k-complexes, spindles, theta) your brain lets off during sleep.
http://en.wikipedia.org/wiki/Binaural_beats
http://en.wikipedia.org/wiki/Binaural_beats#Brain_waves
http://www.binaural-beats.com/binaural-beats/
http://www.binauralbeats.net/
There are more links and things I'm sure but I'm way too lazy to look right now, I will look into them even more tomorrow and try to find you more. It's something to look into I believe.