How come I only have insomnia in summer? Can it be temperature related?

Body temp. Yes, as the body cools down in temperature, the circadian rhythm for sleep is activated. As it heats up, we are wired to be more awake. Try keeping your bedroom cool at night (ideal around 68-70 degrees) and making sure you have light covers but not too heavy.
Agree w Dr. Carnahan. If you are too hot - that will negatively impact sleep. If you have air conditioning on ; you are too cold - that will also make sleep difficult.