That aspect seems to stem from the fact that alcohol increases activity in the dopamine neurons in the mesolimbic reward pathway, as well as opioid cells that release endorphins. Both produce feelings of joy, pleasure, euphoria, depending on the type of activation. That's why drinking can be so pleasurable.
So what gives? As you've undoubtedly noticed, alcohol tends to make you so drowsy you'll pass out on any available surface. According to research, a nightcap does help you fall asleep faster; as long as you stick to just one drink, it's been shown to improve sleep quality and duration.