"Alcohol might help you nod off, but even just a couple of drinks can affect the quality of your sleep"
"Even a couple of drinks can interfere with the normal sleep process. When you drink alcohol close to bedtime, you can go straight into deep sleep, missing out on the usual first stage of sleep, called rapid eye movement (REM) sleep."
In the course of a night you usually have six to seven cycles of REM sleep, which leaves you feeling refreshed. However, if you've been drinking you'll typically have only one to two, meaning you can wake feeling exhausted.