Insomnia Support Group
Insomnia is a symptom, rather than a disease, and is characterized by inadequate quantity or quality of sleep. People with insomnia complain of difficulty falling or staying asleep, which results in significant impairment of daytime functioning. Insomnia is categorized as transient (shorter than one week), short-term (one to three weeks) or chronic (longer than three...
Regarding the nightmares, just speculating, but they typically occur during a REM dream stage, which normally happens after completion of the deeper and most refreshing slow wave sleep stages.
We typically cycle through 4 to 6 complete sets of these stages per night, and awaken briefly between each set. Most people forget about these transient awakenings by morning, but you may have awakened after your second or third set and found it hard to fall back asleep.
In cases like that, our sleep drive is by then significantly reduced, and we rely more on our circadian rhythm, or more specifically our circadian alerting system, which typically is at its lowest ebb in the early morning hours, to finish the remaining cycles.
That's another reason why consistency is so important. Helps strengthen that all important circadian rhythm in addition to synchronizing it to the sleep drive.