Alcohol makes a person feel sleepy primarily because it is a central nervous system depressant that slows brain activity. It enhances the effects of a neurotransmitter called gamma-aminobutyric acid (GABA), which has a calming and inhibitory effect on the brain, thus making a person feel drowsy and relaxed. At the same time, alcohol suppresses excitatory neurotransmitters like glutamate, further reducing brain activity and leading to the sensation of sleepiness. However, while alcohol may help people fall asleep faster initially, it negatively affects overall sleep quality. It disrupts the sleep cycle, particularly by reducing restorative REM (rapid eye movement) sleep and fragmenting sleep patterns later in the night. Alcohol also affects the body's circadian rhythm and causes frequent waking, partly due to its diuretic effect causing more trips to the bathroom. Thus, alcohol-induced sleepiness is often followed by poor-quality sleep and daytime grogginess. In summary, alcohol causes sleepiness by depressing the central nervous system through the action of neurotransmitters, but it disrupts deep, restorative sleep, leading to tiredness after the initial sedative effect wears off.