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Large amounts of alcohol are known to shorten sleep latency, increase slow-wave sleep, and suppress rapid eye movement (REM) during the first half of sleep.

During the second half of sleep, REM increases and sleep becomes shallower. A study of the acute effects of alcohol on the relationship between sleep and heart rate variability (HRV) during sleep has found that alcohol interferes with the restorative functions of sleep.

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Alcohol affects overall sleep architecture," said Yohei Sagawa, a medical doctor in the department of neuropsychiatry at the Akita University School of Medicine. "Normally, during physiologic nocturnal sleep in humans, the parasympathetic nervous system, responsible for ‘rest-and-digest’ activities, is dominant over the sympathetic nervous system, responsible for stimulating activities. We wanted to investigate how alcohol may change this complementary relationship."

"I believe that the approach used in this study is unique," added Seiji Nishino, director of the Sleep & Circadian Neurobiology Laboratory at Stanford University School of Medicine. "Although there are several studies monitoring HRV during sleep, as far as I know there is no report describing the effects of alcohol on autonomic nervous system during sleep using this parameter."

Study on Alcohol and Sleep

"Our study showed that alcohol suppresses the high-frequency power during sleep in a dosage-dependent manner," said Sagawa. "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."

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Alcohol Consumption Increases Heart Rate

More specifically, as alcohol consumption increased, the heart rate increased and the spectral power of HRV measured at each frequency range decreased. Also, the low-frequency/high-frequency ratio that is considered an index of the balance between the sympathetic and parasympathetic systems was increased. This suggests that alcohol, in a dosage-dependent manner, suppresses the high-frequency component of HRV that is an indicator of parasympathetic nerve activity during sleep.

"The current study evaluates the acute effects after only a single dose of alcohol intake, and subsequently found a negative health consequence," observed Nishino. "Many subjects habitually drink alcohol, and if the reduction of parasympathetic nerve activity during sleep chronically occurred, negative health consequences may be much larger and may induce various diseases. It is reported that habitual drinkers with hypertension are often associated with reductions of parasympathetic nerve activities."

Alcoholics Suffer from Insomnia

Sagawa agreed. "Many alcoholics and habitual drinkers suffer from insomnia," he said. "Suppressed parasympathetic nerve activity is the result of alcohol drinking. Thus, it is inferred that suppressed parasympathetic nerve activity is associated with insomnia, which includes difficulty getting to sleep, early-morning awakening, lack of a sense of deep sleep, and difficulty maintaining sleep."

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Sagawa added that it is important for clinicians who are treating physical and psychological disorders related to alcohol to consider the disturbing effects on sleep’s restorative effects that habitual drinking can have.

Reference: Alcoholism: Clinical & Experimental Research.Published Online August 16, 2011.

“Alcohol Has a Dose-Related Effect on Parasympathetic Nerve Activity During Sleep,” Yohei Sagawa, Hideaki Kondo, Namiko Matsubuchi, Takaubu Takemura, Hironobu Kanayama, Yoshihiko Kaneko, Takashi Kanbayashi, Yasuo Hishikawa, Tetsuo Shimizu

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