When you go to sleep your brain’s electromagnetic wavelengths slow from beta to alpha, then down to theta, then down to delta, before shooting back up to beta speed wavelengths as the brain enters REM. So the brain is moving slower and slower and slower and then, in the deepest stage of sleep, the brain is as active as it is during the day, and nobody really knows why. They say that REM is the brain processing the events from the day, though I prefer to understand it as the brain bathing in the collective unconscious.
If you smoke weed or drink alcohol, you do not enter REM the following night. (Popular rhetoric suggests that these drugs suppress REM. And while this may be true of alcohol, I believe that smoking weed steals the brain activity of REM for the waking self. This is why smoking can feel so revelatory, and why I get so extremely tired after the high wears off.) While this is okay every now and then, the brain needs REM to function at its best. Without REM, to sleep is to press pause on the day, and to wake is to press play. There is no reset, no page turn. In REM the brain takes a bath, washing off residual interactions and emotions, turning the events of the day into memory. This way, when you wake up, your slate is clean. The mind is not a closed circuit. It can and must transcend the body regularly in order for an individual to conduct the wavelengths of experience properly.