Alcohol is a central nervous system depressant. It is also a diuretic; it flushes water from the body.
In large amounts (four or more standard drinks) alcohol will make you fall asleep or pass out quickly. This reduces the quality of your sleep, may wake you to go to the toilet frequently, and eventually you wake up feeling less than refreshed!
You will also get interrupted sleep if you consume alcohol or stimulating drugs close to your usual bedtime.
A hangover leads to increased sleepiness and reduced alertness. It can also cause stomach upsets that lead to dehydration and fatigue.
Combining these factors leads to fatigue-impaired performance, especially in high workload and demanding situations such as during inflight emergencies.