my (inexpert) theory is that a loss, a trauma, something just gone very wrong, destabilizes the self/body, so that one's sleep is immediately affected. insomnia is like an illness the anticipation of which--(anxiety)--is almost as bad as the illness.
from insomnia to desperation-to-sleep---this for many must be the crossing-over when one is most vulnerable to addiction.
(absolutely no doubt that this is what led Marilyn Monroe to "overdose"--- she just wanted desperately to sleep & took one sleeping pill too many.)
insomnia/sleep deprivation affects the brain in adverse ways immediately. it is no wonder that many people "self-medicate"--- my wonder is that there are not even more.
literally, many people do not understand addiction; it can seem like a matter of "personal choice" depending on upon your experience. before three people in my life passed away within a few years I did not comprehend how loss can destabilize anyone to the point at which ANY remedy could be a solace, & eventually an "addiction." but it begins with the need, strong as thirst, to assuage a hurt, numb a pain; get through a few hours of the night. no one can judge who has not felt this.