It is definitely related to sleep. Cannabis disrupts NREM sleep, where memories are ushered from short term memory into your hippocampus. Matthew Walker's excellent book "Why We Sleep" goes into this in some detail, highly recommended.
Edit: I should add, many substances disrupt NREM sleep in addition to cannabis, such as alcohol, caffeine, and sleep aids.
Edit: I should add, many substances disrupt NREM sleep in addition to cannabis, such as alcohol, caffeine, and sleep aids.