pls first ask the person who taught you this mantra..
my 2 cents follow ( take them with plenty grains of salt - maybe pennies go down good with salt :-) :
next time u r at ur job, or watching ur favorite TV show, just try to close ur eyes & forcefully go to sleep. how would u feel? restless & angry? the more/longer u force urself, the more ur anger rises?
then u either - stop trying to sleep OR fall asleep after a while.
(mantra = blissful sleep, unresolved issues = job/TVshow)