I always get headaches at night time. What could be causing this?

Anything. Your headaches could be coming from tension, dehydration, lack of sleep/poor sleep, skipping meals, eye strain, teeth grinding, stress, sleep apnea/snoring, or chronic sinus problems.
2 things come 2 mind. 1) cluster type headaches typically occur at almost the same time at night. 2) obstructive-sleep-apnea-triggered migraine.