Coming from a guy who is not a doctor, but i have a BS in biochemistry and also alot of research on this specific topic because i can relate...
Opiates don't actually target the source of the pain like OTC pain killers. They target the dopamine receptors in the brain. The body naturally produces signals to activate these dopamine receptors to make you feel happy and balanced. What opiates do, especially if you're abusing them, is intensify the signals sent to these dopamine receptors greatly. That's why no pain is felt because your brain is sending signals that you're feeling good. So after time, since you're pumping in these signals to your brain, the body stops naturally generating these signals to the dopamine receptors because it does not need to.
So stopping opiates causes a "shock" into the brain because the brain has been accustomed to not sending these signals to the dopamine receptors. When this happens, you feel less happy (hence the anxiety and post w/d depression). Depression and feeling lethargic go hand in hand. It will subside in time. Last time I withdrew I was tired for a good 3 or 4 weeks or so, but not getting sleep, ironic.
Don't dwell too much on it. No one ever died from a lack of sleep. When your body is tired you'll sleep.
Best of luck to you and hope this helps