Always a bad idea. While the newer sleeping meds don't have the deadly synergy displayed in barbituates when mixed with ETOH, you're still mixing depressants that may interact. I'd suggest you quit drinking long before bedtime.
DO NOT mix your diet pills with alcohol. Especially with prescription diet pills. Drinking alcohol or even taking a cough syrup with alcohol in it can interact with the medication.
Examine the situation and try to pinpoint what factor(s) are making it harder for you to sleep, then find ways to resolve these issues. There's numerous reasons why you might not be able to sleep...