I think that Marijuana only works for insomnia on inexperienced users and you can not toke every night (like you would need to, because you need to fall asleep, everyday) and stay an inexperienced toker. I think the reason why it only works on less "seasoned" tokers is because at first being stoned can be a lot for your mind to take in. Marijuana expands the mind with such ease that new thoughts flow like crazy and that it will tire your mind out, called the "burnout".
I know I personally, a longtime, everyday toker, have suffered from insomnia since I can remember and that the only sure-fire cure for insomnia is booze, lots and lots of booze (I am DEFINATELY not condoning drinking alcohol to fall asleep as this is horribly dangerous, as i know).
I also know, for me anyways, that marijuana does not turn my mind off, so sleep is not an option. In fact, I think I do the majority of my thinking when I am stoned.
Marijuana is not classified a sedative or depressant, alcohol is though. The only reason why people experience a period of "tiredness" a while after toking is because they either mentally, physically or a little of the both, exhuasted themselves. After time you learn not to exhaust yourself, otherwise you could never function a whole day after a wake n' bake, lol.