Do you not wake up every four hours with some rebound insomnia and have to redose again?
That's the effect i always get whenever i try to use GBL for sleep.
Probably a good thing in my case because it means i don't even bother trying to use the stuff for this purpose anymore and i do often suffer from insomnia. Even if i do redose after 4 hours to get the full 8 i wake up feeling like i've been abducted by aliens and experienced missing time rather than normally asleep.
If GBL allowed me to get normal sleep that left me feeling rested afterwards i'd probably be addicted by now.