Alcohol causes fatigue and the this fatigue is caused due to consuming alcohol. Because of consuming alcohol the the quality of sleep is reduced and the alertness decreased. It also impairs the performance in a variety of cognitive psychomotor tests.
Here, it is summarized that alcohol intoxication shows high levels of fatigue.
In Nature's rule of law; it is technically both. The only difference between the two would be photosynthesis happens with plants. While cellular respiration happens with living beings.
