When you sleep you usually go through two alternating states, characterized in part by different types of brain electrical activity (i.e., brain waves). These states are described as slow wave sleep (SWS), in which brain waves are very slow, and rapid eye movement (REM) sleep, in which the eyes undergo rapid movements although you remain asleep. Most sleep is the deep, restful SWS. REM sleep occurs periodically, occupying about 25 percent of sleep time for most young adults.

Alcohol consumption can affect both SWS and REM sleep in many different ways, causing many different problems, from general fatigue, to illness, to falling asleep at the wheel and causing a traffic accident.

If you have a nip of alcohol at bedtime, it may actually help you to fall asleep - provided you don't do it too often or the sleep-inducing effect wears off. However, studies show that alcohol consumed within an hour of bedtime can lead to fitful sleep.

The same is true for alcohol consumed in the late afternoon, for example at “'happy hour” or with dinner, without further consumption before bedtime. Studies show that a moderate dose of alcohol consumed as much as six hours before bedtime can increase wakefulness during the second half of sleep, even though the alcohol has already gone from your body by that time.