there are a lot of articles on people who wake up at 3am - apnea, exercise (too much / too little), stressed about something and affects you subconsciously, lifestyle choices, inconsistency in sleep pattern, melatonin disruption etc 
https://www.healthline.com/health/sleep/why-do-i-keep-waking-up-at-3-am#sleeping-tips
https://youtu.be/PpIg5V0QBWQ