I don't think that you have actual "insomnia" - it's probably a reaction to trying to sleep during the daytime, which most people cannot do very well, if at all. If you could sleep well at night before this job, you're not an insomniac.

I wouldn't recommend working nights long-term. I did, and almost cracked up - in spite of the fact that I'm a lifelong "night person", typically staying up past midnight, and was able to sleep during the daytime. Some companies with night shifts are aware of how unhealthy working nights is, and rotate their employees' shifts periodically.