Why did stopping antidepressants cause insomnia?

I stopped taking antidepressants a while ago. After two weeks I couldn't sleep all night and then wouldn't be tired in the day. My doctor was mystified as prior to that my sleep had been really good. I also noticed that it was only happening on certain days of the week. This led me to think that it might be something I was eating, and I think I have become intolerant to refined sugar.