By guest blogger Julia Gottwald

The last time you and your class-mates or co-workers pulled an all-nighter before a deadline, you may have noticed: there are always those lucky individuals who seem to do just fine after a lack of sleep, while others feel drowsy and confused – almost like they had too much to drink.

New research conducted at the German Aerospace Center suggests this could be because alcohol intoxication and sleep deprivation are more similar than we once thought.

In their study published recently in PNAS, Eva-Maria Elmenhorst and David Elmenhorst and their colleagues show how both affect us via a shared mechanism. And what’s more, if you’re sensitive to one, you’re likely to cope poorly with the other as well.

The researchers tested the ability of 49 healthy volunteers to sustain their attention over a long period of time. The tests took place on four different days, under different conditions: baseline, after drinking alcohol, and after either partial or full sleep deprivation.

Sustained attention was measured as reaction time on the 10-minute Psychomotor Vigilance Task, in which participants had to press a button as fast as possible in response to a randomly occurring light that appeared on-screen. On test days the task was repeated every 3 hours.

For the alcohol condition, the volunteers had to drink enough vodka to reach a blood alcohol level of at least 0.06 per cent – over the drink drive limit for most European countries. Some participants even reached levels of almost 0.1 per cent.

When the researchers then looked at how much the participants’ reaction times were slowed down by alcohol compared to baseline, they saw big differences between them. But crucially, this wasn’t explained by the level of alcohol in their blood. Some participants with the highest alcohol levels had almost no impairments, while others closer to the lower limit had much slower reaction times. You may have witnessed a similar phenomenon with your friends. Even though you’ve all had the same amount of drinks, some can have a perfectly normal conversation, while others can’t even build a clear sentence.

The researchers then split the group into two – those with low or high sensitivity to alcohol – to see if the same groups would be more or less sensitive to sleep deprivation as well.

Indeed, after either 35 hours of total sleep deprivation or 4 successive five-hour nights, the participants most adversely affected by lack of sleep were also the ones who had been most sensitive to alcohol.

On a molecular level, this similarity in response to alcohol and sleep deprivation had previously been shown in animal models and cell cultures. Both alcohol and sleep deprivation affect the adenosine system, which could help explain the performance differences between participants.

Adenosine is a molecule formed during normal energy metabolism and which accumulates in the brain through the day. By binding to its receptors, of which the A 1 adenosine receptor is the most common, adenosine makes us sleepy. When we are sleep deprived, more of these receptors become available on cell surfaces, thus giving adenosine more binding sites, which makes us even more sensitive to the molecule.

In a similar fashion, alcohol increases the concentration of adenosine and thereby induces sleepiness. But what hadn’t been shown before this study is how alcohol affects the A 1 adenosine receptor in the human brain. By using a kind of brain scanning called positron emission tomography (PET) with more volunteers, the researchers showed that alcohol, like sleep deprivation, increases the availability of A 1 adenosine receptors, in a similar way to sleep deprivation.

This study therefore provides evidence that alcohol and sleep deprivation affect the adenosine system in very similar ways, and that personal differences in this system likely contribute to the way our sensitivity or resilience to both manifests as an individual trait (although the full picture is more complicated – sensitivity to alcohol, for example, is known to depend on a number of factors and has been linked to several genetic variations).

Though these results are important, they have several limitations. The volunteers underwent more experimental conditions than included in the key analysis of the effects of sleep deprivation and alcohol intoxication. In fact they spent a total of 12 days in the sleep laboratory, including an adaptation night and other challenges, including exposure to air low in oxygen or alcohol followed by partial sleep deprivation. Although these conditions were always followed by recovery nights, there may have been some carry-over effects that influenced the currently discussed results.

The study samples were also relatively small and uneven (for instance, of the 47 volunteers from the total sleep-deprivation group, 12 could not complete the alcohol condition, and only 14 participants completed the partial sleep deprivation condition). The results should be replicated using bigger samples to confirm that sensitivity to alcohol and sleep deprivation is a true individual trait.

Moreover, although sustained attention is a key function, it would also be interesting to look at other cognitive functions next. If the individual trait “sensitivity to alcohol and sleep deprivation” exists, does it also affect abilities like memory or problem-solving? These measures would give us a deeper understanding of how volunteers will cope with their daily tasks after having a drink or a short night.

As a first step for yourself, it could be useful to be more aware of how your body reacts to alcohol and sleep deprivation, and especially to consider if you are sensitive to both. For example, if you’re the kind of person who wouldn’t dare give an important presentation, or drive a car, even after just a single, weak drink (because you know your own sensitivity to alcohol), this is a good indicator that you probably shouldn’t undertake such challenges after a short night either. Taking sleep seriously can be hard, given that external pressures and stress cannot always be influenced. But being aware of the full impact of sleep deprivation can be an important first step for a healthier lifestyle.

—Cognitive impairments by alcohol and sleep deprivation indicate trait characteristics and a potential role for adenosine A1 receptors

Post written by Julia Gottwald (@julia_gottwald) for the BPS Research Digest. Julia is the co-author of “Sex, Lies, and Brain Scans”, which won the BPS Popular Science Book Award 2017. She completed her PhD in psychiatry at the University of Cambridge and also holds degrees in neuroscience from the University of Oxford and biochemistry from Free University of Berlin.