Someone else posted actual research stuff a few pages back, but here's a different study that was done slightly differently.
Researchers have demonstrated that THC ingestion decreases SWS (Slow-wave sleep) and REM sleep, and has sometimes been found to eliminate REM sleep altogether in rats, rabbits, and cats. In later research, the same effect was observed in humans in controlled studies, with the added finding that sleep cycles did not return to normal until after about one week of abstinence.
And Sleep cycle descriptions:
Just prior to falling asleep, the typical person is in a relaxed state of consciousness characterized by alpha waves (a frequency of 8 to 12 waves per second). After the person falls asleep, phase one of the sleep cycle begins, during which brain activity is still fairly high but declining. When a person reaches stage two, brain waves become slower. The third and fourth stages of sleep are known as slow-wave sleep. During SWS, heart rate, breathing rate, and brain activity slow down and the percentage of slow, large-amplitude waves increases. After stage four, a person cycles back through stages three and two. However, instead of returning to stage one, the person enters a stage of sleep known as rapid eye movement (REM) sleep.
You can make your own conclusions about what that information means, they're just excerpts from a few scientific articles I had from psych last year. Seems pretty clear to me, although it doesn't really show much for long term use and possible adjustment I suppose.