A nightcap may be nice, but is likely to lead to a fragmented night's sleep.
While it is true that alcohol acts as a sedative, it also disrupts dreams and shakes up the sleep cycle. It reduces time spent in the stage of sleep understood to be the most restorative, the rapid eye movement, or REM, phase. Prolonged use of alcohol can cause insomnia, sleep apnoea and snoring.
These are the findings of the London Sleep Centre which has published a review of all known studies on the effect of alcohol on sleep in healthy volunteers.
''At all dosages, alcohol causes a reduction in sleep onset latency, a more consolidated first half sleep and an increase in sleep disruption in the second half of sleep,'' the authors said in the latest issue of the journal Alcoholism: Clinical and Experimental Research. Lead researcher on the review, Dr Irshaad Ebrahim, told the BBC people should be cautious about using alcohol to send themselves to sleep.
''One or two glasses might be nice in the short term, but if you continue to use a tipple before bedtime it can cause significant problems,'' he said. ''If you do have a drink, it's best to leave an hour-and-a-half to two hours before going to bed so the alcohol is already wearing off.
''With increasing doses, alcohol suppresses our breathing. It can turn non-snorers into snorers and snorers into people with sleep apnoea - where the breathing's interrupted.''
Professor Peter Cistulli, the head of sleep medicine at the University of Sydney and director of the Sleep Health Foundation, says there are three reasons for this.
First, alcohol is a muscle relaxant so the throat muscles become vulnerable. Second, the additives in some forms of alcohol, particularly red wine, cause nasal congestion. This means people are more likely to breathe through their mouths and therefore more likely to snore and more prone to sleep apnoea. Third, ''alcohol numbs the brain [so, if for instance] the throat collapses and there is alcohol on board, recognition of that problem is delayed''.
From the hundreds of studies assessed by the London Sleep Centre, the most significant finding was the effect of alcohol on REM sleep. This is because the body does not slip into the deeper dreaming sleep state until it has metabolised the alcohol.
''The onset of the first REM sleep period is significantly delayed at all doses and appears to be the most recognisable effect of alcohol on REM sleep followed by the reduction in total night REM sleep,'' the researchers said.
REM sleep is still not fully understood, but it is seen as necessary for survival. Studies on rats, for instance, have found that those deprived of REM sleep survive only about five weeks on average. Rats normally live for two to three years. Reduced REM can also lead to people feeling more fatigued the next day. One British study found almost half of the 2000 drinkers surveyed acknowledged increased tiredness after a drinking session, but 58 per cent did not realise alcohol was the reason. Cistulli said a greater awareness of the effect of alcohol on sleep means people are more able to address sleeping issues.
''Alcohol is a chemical and the brain is a soup of chemicals,'' he said. ''Alcohol gets in there and mucks up the soup that is relevant to sleep.''
However, he said the London Sleep Centre review is based on generalisations and ''there are clearly individual variations''. The findings are of most benefit to people who have trouble sleeping and aren't aware of the impact alcohol is having, Cistulli said. ''Once they start to understand the link, they can start to modify their behaviour.''
For more information about how to get a good night's sleep, go to: sleephealthfoundation.org.au