according to Dr. Gerhard Eggetsberger (an Austrian scientiest) using symbols for neuro-programming prior sleeping is very effective. the science behind it: approx. 20 minutes before sleeping the Thalamus shuts up. The Thalamus is the censor in our brain. So when this censor becomes silent suggestive formulas (wether symbols or/and mantras) easily slip into the subconscious and become powerful over time.

here you can read the articles (sorry just in german):
Powergedanken - De - Hypnose
Symbole - De - Hypnose

i have been having the same idea but didnt try it yet. but soon

good luck and report us back!
Cheers