I've heard it mentioned before that melatonin is depleted by cannabis, or rather the rate at which the brain secrets the hormone is negatively impacted.
Oddly enough, for this theory it makes rather good sense to me, as melatonin is directly related to almost all the known common issues of heavy cannabis withdrawal due to sudden stoppage, such as: sleep issues regarding the circadian clock (regardless of external cues - or rather, independent of), limited memory or temporarily limited ability to produce new information in the cerebral cortex, impaired motor function (in general), and issues sudden drops in activity in the temporal lobe - the area responsible for processing and interperating language.
The real question, however, is to what degree cannabis actually effects melatonin, or if it's simply the disruption of our normal circadian rhythm from the sudden stoppage of cannabis use (AKA sleep deprivation - which shares each and every symptom listed above to the T). It's probably more of a domino effect than any one thing - a drop in melatonin secretion, again, a necessary sleeping hormone naturally used by in the brain as a cue for a need for slow-wave delta-type sleep (EEG), would certainly explain the disruption in sleeping regulation - and thus the effects of sleep deprivation set in.
I'm positive there are studies out there that address this directly. I would venture to guess that simply taking melatonin supplements would be highly effective during the same period of time you plan on ceasing cannabis use would counteract a majority of these effects - melatonin production should not be permanent effected in any way from cannabis use.
I don't have the time right now, but if anyone could find an abstract, I don't think it would be difficult to locate many articles and studies addressing this question. You can try PsycINFO or PsycARTICLES - search primary cannabis or marijuana in the title, along with melatonin in the abstract, and that should yield a ton of results.
IME - Melatonin in large doses, used for short stretches, aids greatly in sleep regulation. I use Melatonin and Velarian Root, both OTC nautral supplements, to aid in sleep when I have issues getting decent rest. Both should help you reach not only stage 1 sleep but also REM and delta wave stages of sleep (3-4).