Marijuana reduces REM sleep slightly in the short term, therefore reduces sleep disruption. However, continuing use of the substance produces a sustained reduction in SWS (Slow Wave Sleep).
http://www.portfolio.mvm.ed.ac.uk/st...oup24/what.htm

Marijuana ingestion has minimal sleep disruption with slightly reduced REM sleep acutely, however chronic intake causes long-term slow wave sleep suppression.

Alcohol -Acute ingestion of alcohol even in low doses causes changes in sleep. When blood alcohol levels < 10 mg/dl â?? increased total sleep time and reduced awake activity.

http://www.portfolio.mvm.ed.ac.uk/st.../31/stages.htm