Hi Elsie,
If the Mirena is helping control your insomnia, then I think you have to weigh up for yourself whether you can live with the spotting or not.
I had a Mirena fitted in 2012 for heavy, flooding periods and I had constant spotting and light bleeding with it - but at the time decided that the controlled, light bleeding was preferable to having to always have a change of clothes handy 'just in case'. I added in Elleste Solo and later changed to Estradot 25 but was still bleeding almost continually (I would have a week or so now and again without any bleeding). Eventually I stopped the Estrogen and had the Mirena removed in 2014 because I was so tired of constantly having to wear panty liners. After that I would have heavy bleeding for 2-3 days during a period but my periods were erratic, so it was not every month and I decided that was the best for me at that point. My doctor said Mirena stops periods in most women - I guess we are just part of the unfortunate minority.