You may never know.
A number of studies have shown that drugs like Ambien and Lunesta offer no significant improvement in the quality of sleep that a person gets. They give only a tiny bit more in the quantity department, too. In one study financed by the National Institutes of Health, patients taking popular prescription sleeping pills fell asleep just twelve minutes faster than those given a sugar pill, and slept for a grand total of only eleven minutes longer throughout the night.
If popular sleeping pills don’t offer a major boost in sleep time or quality, then why do so many people take them? Part of the answer is the well-known placebo effect. Taking any pill, even one filled with sugar, can give some measure of comfort. But sleeping pills do something more than that. Drugs like Ambien have the curious effect of causing what is known as anterograde amnesia. In other words, ingesting the drug essentially makes it temporarily harder for the brain to form new short-term memories. This explains why those who take a pill may toss and turn in the middle of the night but say the next day that they slept soundly. Their brains simply weren’t recording all those fleeting minutes of wakefulness, allowing them to face each morning with a clean slate, unaware of anything that happened over the last six or seven hours. [More]