Insomnia is a medical condition. In many places, medical conditions which do not require special care from the employer are a taboo topic in job interviews. Even in those where the law does not forbid questions about medical conditions, you are better off not mentioning them.
If the question comes up (most companies have trouble filling the night-shift positions and are glad for anyone they can get), just say:
I am a night person
Some people simply have an atypical circadian rhythm which makes them sleepy during the day and active at night (some people call this a disorder, which I find discriminating). Those people are perfect for night-shift jobs.
(This often gets misdiagnosed as insomnia, by the way. Insomnia is when you can not sleep well at all. But people with atypical circadian rhythms can sleep perfectly fine when allowed to follow their natural sleeping pattern. It only looks like insomnia when they are forced to wake up early every morning).
Another concern you might want to diffuse is that you don't have a day-job. Some people are so hard on cash that they need both a full-time job during the day and a second job at night. Those people often have trouble getting enough work-life balance to do either job properly.