Disturbance of the deep/light sleep cycle
Alcohol seems to accelerate falling asleep, at least in subjects who do not tend to fall asleep immediately. On the other hand, the negative effects arise later and affect the quality and duration of sleep. Sleep is a complex process, characterised by alternating phases of deep sleep, known as paradoxical or REM sleep, during which the subject dreams, and slow wave sleep. Undisturbed progression of these two phases of sleep is essential for an individual's well being. Alcohol disturbs or interrupts the sequence of paradoxical sleep and light sleep. Thus alcoholics and some people who have stopped drinking complain about disturbed and fragmented sleep, frightening dreams and insomnia.

The negative effects of alcohol last well into the night, even when the alcohol has been eliminated. This phenomenon is not specific to alcohol and is seen with other sedative products. Snoring is abnormally frequent after consuming alcoholic drinks in the evening before going to bed. This is due to the relaxing effects of alcohol on the pharyngeal muscles.

Daytime repercussions of the effects of alcohol on sleep
Disturbed sleep or sleep deprivation exacerbate the sedative effects of alcohol during the day. Alcohol consumed late in the evening will noticeably reduce the performance of a subject (attention, dexterity,...) during the early hours of the following morning. By producing an accumulation of nights of poor sleep, alcohol can disrupt the normal sleep/wake cycle, which is also essential for an individual's well being. Hence the negative effects of alcohol on sleep have repercussions on the performance of the subject during the daytime.

Alcohol and attention
The sedative action of alcohol affects attention to variable degrees, always reducing it and producing diminished performance. This action is particularly noticeable in subjects who lack sleep or who tend to be lethargic. Alcohol seems to reduce the ability of an individual to wake up, even if consumed in moderate amounts, to the point where driving ability is affected, not just in the hours after consumption, but sometimes for days afterwards (see Alcohol and driving).