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There's the zombie effect. Ambien and its generic, zolpidem, work by latching onto a brain molecule, the GABA-A receptor, that dampens brain activity, making you sleepy. The effect is so strong in some people that when you rouse in the night (say, to roll over), the prefrontal cortex, which controls planning and self-restraint, keeps snoozing. So your body's active but your conscious brain isn't, and behaviors like sleepwalking are more common. Some Ambien-related mishaps border on funny, like the woman who unknowingly ate an entire 9" x 11" pan of tuna casserole. But some people have reported doing scary stuff while on zolpidem, like sleep driving and waking up behind the wheel of a crashed car. Not that they necessarily remember—Ambien can also cause amnesia: When it turns on the GABA-A receptor, the brain may have trouble forming new memories.
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And it may hit us harder. In a study involving both sexes taking the same dose of zolpidem, women had much more of the drug in their bloodstream, and it stayed there longer. No one knows exactly why. But one study found that the morning after taking zolpidem, some women were so groggy, their driving was technically worse than if they had been legally drunk—one reason the FDA halved the recommended dosage for women.
The bottom line. Most people tolerate the drug with no behavioral mishaps, says the FDA. But there's no way to know if you'll be the exception, and the risk of potential hazards may remain. The craziest thing: For all their drawbacks, drugs like Ambien help people fall asleep on average just 13 to 17 minutes faster and increase total sleep time by only about 11 to 32 minutes a night. Warm milk, anyone?