Whats the main cause of insomia?

There is no #1: cause for insomnia. Alcohol, caffeine & other stimulants can keep one awake. Exercising or eating a meal too close to bedtime. Too much light in a room or a room that is too hot or too cold are other causes. Fretting anxiously when on lies down keeps the brain overly stimulated. Noise is a problem. Many medications can cause sedation. etc, etc, etc. Uses good sleep hygiene.

Probably stress: Most people experience problems sleeping if they are worried or stressed about a situation or problem in their life. Usually this gets better when the problem resolves. Other people develop more chronic insomnia from chronic worry or stress, depression, or a sleep disorder. Also certain meds can cause insomnia, and make sure you don't exercise or have caffeine for a few hrs before bed.