>>10912683Because one of the things sleep affords your brain is time to go through the days memories and file things.
If you had a busy day with a lot of sensory input your mind is going to be actively archiving it.
I know every time I have to drive a lot (like a road trip, or running a shitload of errands around town) I have super anxious fever dreams about driving.
Most of your dreams during the average sleep phase are really mundane slice-of-life kind of dreams that are basically replaying random chunks of your day. Towards the end of the REM cycles the dreams tend to get weirder and more abstract, and since this is when you usually wake up naturally its the tail end of the weirdo dreams you remember.
If you did a bunch of reading or studying, your brain is going to try and replay that, but writing doesn't translate into dreams, so the sliver of your awareness that's not completely asleep is being slapped in the face with the fact that something is off, which pulls you into a semi lucid hypnagogic state.
