Because Lucid Dreaming and MJ is such a subjective topic, here is a study which provides support that MJ helps with sleeping - not DREAMING.

I just want to let you know what when you feel refreshed in the morning, sleep brain waves in slow wave sleep are that which allow body to regenerate, NOT REM. REM is for mental organization and dreaming.

http://norml.org/index.cfm?Group_ID=6057

Alcohol brings you to FALL asleep faster but takes longer to get deeper in REM.

Once again, MJ is used for medicinal purposes by reducing chronic pain so people can get to sleep. BTW, more MJ smoking also means more lung wrecking and thus - create respiratory problems that can result in awakening in the night and therefore less steady sleep.