I don't have insomnia, but I have a lot of trouble sleeping. It's been a long time since I experienced that wonderful feeling of true exhaustion. The kind where you long for your bed and actually wake up feeling rested in the morning.
For me, it's the opposite. I have to force myself to go to bed, only to fall asleep after about two hours of non-stop thinking. I don't need to explain how I feel when I wake up.
You ask a really good question. In my case, I know where this comes from.
Your questions seems so simple but is yet so important for everybody to ask themselves.
A Good night's sleep is so important for restoration, healing, productivity and mental health. It's good that you are taking interest in the subject.
Depending on your age, nutrition, habits and life events, a racing mind or trouble calming the mind can also be a number of factors.
For instance, someone who has experienced severe trauma might have trouble controlling the thoughts that invade the mind. The stress will raise cortisol levels and adrenaline, putting them into a sleepless mode.
A common contemporary habit is screen time. Watching movies or engaging online interests exercises the brain when it's time to slow down and not focus on anything important. The light from TV, phone or computer screens activates the pineal gland to say it's morning and time to wake up.
Another common cause is dietary.
Magnesium and calcium deficiency is present with most people. I will share this class with you about the details that might be helpful.
I hope these common possibilities are helpful.
Blessings