Alcohol suppresses your central nervous system and increases the production of adenosine—a chemical that makes you feel sleepy. This can make you feel relaxed and help you fall asleep.
However, after a few hours, the adenosine production will drop and your body will begin to remove the alcohol from your system, drawing water from your cells and flushing it out along with essential electrolytes. This loss of fluids, electrolytes, and disruptions to sleep rhythms is coupled with alcohol’s ability to block REM sleep. This means that even though you may fall asleep faster the quality of your sleep will be greatly compromised.
Even a single glass/serving has this effect. It takes many hours for your liver and kidneys to degrade and excrete alcohol — the best advice is either abstinence or drinking earlier in the evening 3-4 hours before bedtime.