When i wake up in the middle of the night, i write in my dream journal right? Then when i go back to sleep, it seems like as soon as i fall asleep, i’m plunged into another dream. My question is, does the sleep cycle start over once you awaken, or does it pick right back up where it left off? 
Well your brain seems to keep a balance between REM and Deep sleep. Say if you only had Deep sleep one night, the next night You’ll have Alot of REM sleep.
But walking up half way though a REM period would probably make you go back into a dream. Later in the night though, you have alot more REM than Deep sleep.
I woke up at 4 AM last night, I could remember my dream pretty well. I fell asleep again and ended up in the same dreams as previously. I woke up at 8 AM and fell asleep again, had the same dream again and woke up at 10 AM.
Now, this was the first time I ended up in the same dream as previously.