Nightcaps may indeed help you fall asleep faster, but it's going to hurt your quality of sleep. Alcohol can cause you to awake frequently in the night, reduce the amount of deep sleep you get, and cause you to wake up earlier than usual in the morning. The overall effect is that you end up feeling tired the next day. And thus you might be tempted to use alcohol to fall asleep the next night and the night after that and so on.
Strikingly, a large number of recovering alcoholics who participated in a study on insomnia in recovery from alcoholism claim to have had problems sleeping for some years before their alcohol dependence developed. And it's an established fact that trouble sleeping is also one of the major triggers of relapse among recovering alcoholics. Recovering alcoholics have difficulty both falling asleep and staying asleep, and this is a serious problem for people who are trying to recover from alcohol dependence. Though there is not enough data at this point to establish a cause-and-effect relationship between insomnia and alcoholism, the connections between the two are noteworthy. It's enough to make you reconsider using alcohol as a sleep aid.