In a recent study, combining moderate alcohol consumption (within legal limits for driving) and moderate sleep restriction led to greater drowsiness and increased deficits in attention, compared with either sleep restriction or alcohol intake alone.

The synergistic effects lasted between 2 and 3 hours. Also, peak impairment occurred not at peak alcohol levels but 30 to 60 minutes after, despite receiving rest intervals in between testing.

The findings suggest that individuals who have previously had inadequate sleep should refrain from performing safety critical tasks for several hours after the consumption of alcohol, even when their breath alcohol content is lower than that required by law. As performance impairment is still apparent with frequent breaks, breaks without sleep should not be seen as safety measures to mitigate impairment.

"Many individuals may make a decision to drive based on the amount of alcohol consumed and the time since consumption. Our data suggest this represents an unperceived risk following inadequate sleep, since performance impairment remains many hours after alcohol was consumed," said Dr. Clare Anderson, co-author of the Human Psychopharmacology study.