Your body has become tolerant to the drugs, it sounds like. You may be running the risk of developing a dependency on the drugs and may suffer withdrawal symptoms if you try to stop.
This is quite common with long term use of tranquillisers or sleeping pills.
You should give this page a read from the national institute of health http://www.nlm.nih.gov/medlineplus/d...r/a605009.html
Hope your sleep improves soon.