Marijuana

Contrary to popular belief, marijuana is not helpful for sleep.

Across research studies, giving people THC (i.e., active ingredient in marijuana) for sleep yields mixed results:

• THC helped some people fall asleep but was arousing and sleep disruptive for others.
• THC may increase one particular stage of sleep (i.e., N3 sleep = deep, slow wave sleep), but contradictory findings have been observed.
• One consistent research finding is that marijuana use decreases REM sleep, which stimulates the brain regions used in learning.
• Marijuana can also increase sleep fragmenting (i.e., cause increased awakenings), flatten circadian rhythms, and increase daytime sleepiness.
• Even if marijuana helps you fall asleep initially; tolerance to this effect and dependence occurs with frequent, chronic use. 

Source: Cannabis, Cannabinoids, and Sleep: a Review of the Literature. Current Psychiatry Reports.