Depending on where you live, I can think of 3 reasons why your alarm would often be triggered during the night:

1. Cats - they like to climb on cars, plus the hood will have residual heat (nice and warm). Some mothballs around the car will discourage them.
2. Raccoons - nocturnal and always looking for food. Do you leave any food inside? Raccoons are habitual and will return at about the same time every night. Remove all food and park in a different location for a few nights to change their habit.
3. Supersonic planes. Do you live along the Northeast flight paths? The Washington and New York to Europe flights fly along the coast up to Canada before crossing the Atlantic. Military aircraft fly early in the morning in many areas, when commercial traffic is light; do you live within 100 miles of an airbase?