It also fragments your sleep. You will wake up many more times a night than if you weren't drinking. Those awakenings tend to be quite brief and many people don't commit them to memory. So they wake up the next morning and feel unrefreshed and not particularly chipper. And they never connect the dots, which is perhaps it was the glass of red wine I had before bed and it was the multiple awakenings that I had throughout the night that makes me feel this sluggish and underslept the next day.