These words are similar but not the same.
Wakefulness: the state of not being able to sleep. It refers to occassions when you can't sleep. For example you can speak about a wakeful night, maybe because there was some noise or something that caused you this state.
Insomnia: It's a medical condition, so it gives the idea of something that usually happens and that needs treatment.
So, wakefulness may be a symptom of insomnia, but not necessarily.