Consequences of Singletons

Question

So I just delved into the Singleton classes and yes, I find them quite helpful. I use my singletons mostly for data storage for multiple targets (views, tables etc.). That being said, I can already see myself going to implement a lot of singletons in my project.

But can a lot of singletons have a negative impact? From what I've read about singletons is that you create one instance for each of them in a proces. Other class instances get released (assuming they get released properly) from memory, then should singletons be released too?

So to narrow it down to one question: Is it harmful to have a lot of singletons?