Nowadays, Americans average less than the recommended 8 hours of sleep per night. A nightcap, or drink taken before bed, is thought by many to help with sleep. However, alcohol actually interferes negatively with sleep as your body processes it. Throw an energy drink mixer in and it is chemically worse news for your rest. "Just as with caffeine, your brain proves remarkably adept at adapting itself and responding to the ethanol molecules jamming up its receptors and interfering with neuron firings. It takes a bit for the brain to catch up, though." (Braun's Buzz via Lifehacker)
The brain's receptors and pathways try to reclaim equilibrium, which can wreak havoc on general resting and on crucial REM sleep. REM stands for rapid eye movement - it is the stage of sleep during which the brain gets its most critical rest and rejuvenation. If you've had caffeine, too, this adds to the situation. Caffeine is a drug that can take up to 5 hours to break down half a dose. Throw caffeine into the mix (as many fans of vodka Red Bull mixed drinks do) while your brain is trying to compensate for alcohol, and the combined "revenge" of both drugs can lead to some fairly restless sleep.