Have you changed anything in your diet or supplement routine? It's possible a new food or supplement is causing your insomnia. Allergies will do that too and we have very high pollen counts where I live. Have you undergone stress ... severe illness in yourself or family member? Do you have any other health issues?
Adrenal fatigue can cause a rise in evening cortisol levels that interfere in sleep. Undiagnosed thyroid issues very often lead to adrenal issues. The early stages of adrenal fatigue/insufficiency is usually high cortisol as the body reacts to stress.
If stress is the issue, a 24hr saliva cortisol test will give you an idea of what your cortisol levels are a 4 times during the 24 hr period and if your midnight reading is high, insomnia will often be a reported symptom. It was with me.