In his podcast, he talks about that with Seth Rogen. Turns out that marihuana as well as alcohol induces sedation rather than sleep.
This causes the brain to withhold REM sleep until the sedative has worn off, then suddenly catches up. So lots of people report very vivid dreams at the end of the night.
Since the amount of REM sleep was less, the quality of the sleep is also lower (he said only half or so).
