Arista,

I used to get exercise induced insomnia when I would do intervals at night. Back then the only problem I had was in falling asleep (would stay up an extra 2 hours past bedtime but that was it).
What I am going through now and you may be experiencing a touch of is adrenal stress. Your body starts spitting out cortisol at all the wrong times. Cortisol is a stress hormone. The drop in cortisol at night is what causes the crash when you fall asleep and low levels of cortisol help you stay asleep during the night.
I had a hard time believing that my ride early in the day triggered my insomnia. But it was true. If I worked out during the morning. I'd feel fine all day but by night time my cortisol would pitch up. I couldn't fall asleep and would wake up every two hours (2AM and 4AM religously). When I was in my worst state, I would wake up in middle of night with my heart racing, rapid breathing and sweating (almost like hot flashes). I woke up as if startled by something and then couldn't get back to sleep. This would go on a few days after the ride. I tested this by staying off the bike, and then my body would normalize a bit but as soon as I went out there and pushed it again, my overworked adrenals would respond with excess cortisol and I'd be right back in that vicious insomnia cycle.
Be careful. Once your cortisol/adrneals go, it is prone to mess up your other hormones as well. Make sure you are properly hydrated and are getting sufficient calories (especially when you are riding). Take your recovery seriously. If you feel tired, don't push it. It will cause your adrenals to work very hard to overcome the fatigue it feels in the face of additional stress from training.
I've gotten used to taking my resting morning HR. If it is 5 points above normal, I either stay off the bike or scale my ride way back. The times I didn't adhere to this rule, I noticed I suffered insomnia that night.
Hope that makes sense.