Relaxation is a recommended technique to help us to sleep ... so why isn't a night cap a good idea? Alcohol actually helps induce deep sleep, the problem is that the beneficial effect last only about 4 hours, and then you get the alcohol withdrawal effects with the sweating, the rapid heart beat, the waking and anxiety. Studies showed that the detrimental effects begin after 2 drinks.
Alcohol suppresses REM sleep (Rapid Eye Movement sleep), which is the restorative sleep necessary for many important functions. There is a greater percentage of REM sleep in the second half of the night, and alcohol disrupts the second half of the night more than the first half.
Keep in mind that alcohol increases snoring and worsens sleep apnea significantly. Given the enormous incidence of undiagnosed sleep apnea in the general population, this is another reason to be careful about having too many drinks before sleep.
Finally getting into the habit of drinking oneself to sleep leads to alcoholism.
Alcohol also causes acid reflux by opening the lower esophageal sphincter (the stomach valve), and cause awakening and arousals related to acid reflux.