Habit-forming humans. Once a behaviour is ingrained, it's hard to change. If you once had to wake up at 3 a.m.
Different sleep phases cycle through the night. Which phase you're in affects spontaneous waking.
Interruption in your breathing while you sleep (sleep apnea) could be the cause of your 3 a.m. awakenings.
According to the U.S. National Library of Medicine, older people spend fewer hours in deep sleep and awaken three to four times per night (NLM).
Even though a drink or two before bed might help you relax at first, it can mess with your sleep and wake you up in the middle of the night.
Having trouble sleeping at 3 a.m. could be a circadian rhythm disorder. Circadian rhythm disorders, also called sleep-wake cycle disorders
According to the Mayo Clinic, people with nightmare disorders may have nightmares several times a night.