If you're coming off alcohol, there's something called post acute withdrawl syndrom. It's after you're over the initial physical withdrawl as your brain adjusts to life without chemicals. Effects include short term memory loss, and difficulty with concentration. It took me about 6 months after I stopped drinking for my brain to feel like it was back to normal. Don't know if there's an equivalent for other drugs. And yes insomnia is pretty common after you stop using.

I find drinking a relaxing herbal tea right before bed helps.

--splitimage