Alcohol is a poor nightcap and robs sleep of its restorative powers, research has shown.

A study of 10 male university students found late drinking had effects on the nervous system that promoted sleepless nights and exhaustion.

Participants were given a series of alcoholic drinks of different strengths at three-week intervals.

Each was asked to drink one of the beverages 100 minutes before going to bed. The alcohol content of the drinks ranged from zero to one gram of pure ethanol per kilogram of body weight.

Electrocardiogram and electroencephalogram readings were taken, showing electrical activity in the heart and brain.

Alcohol was found to suppress parasympathetic nerve activity – the part of the “autonomic”, or non-voluntary, nervous system that slows heart rate and aids digestion. Known as the “rest and digest” nervous system, the parasympathetic nerve circuit helps maintain and restore energy.

The findings were published in the journal Alcoholism: Clinical & Experimental Research.

Study leader Yohei Sagawa, from Akita University School of Medicine in Japan, said: “Although the first half of sleep after alcohol intake looks good on the EEG, the result of the assessment regarding the autonomic nerve system shows that drinking leads to insomnia rather than good sleep.”

Colleague Seiji Nishino, director of the Sleep & Circadian Neurobiology Laboratory at Stanford University in the US, said: “It is generally believed that having a nightcap may aid sleep, especially sleep initiation. This may be true for some people who have small amounts of alcohol intake.

“However, it should be noted that large amounts of alcohol intake interfere with sleep quality and the restorative role of sleep and these negative consequences may be much larger during chronic alcohol intake.”