I just heard this today, and the reason is because some women ( a lot of them) metabolize ambien slower than expected, thereby making them think they are ok when they wake up, when really, they still should not be driving.

For some like me, we metabloize medications very quickly. I really hope that my 12.5mg ambien cr is not forced to be reduced to 6.25 because I know 6.25 is not enough.