Ingestion of ethanol just prior to sleep onset impairs memory for procedural but not declarative tasks

The aim of Experiment 1 was to determine if moderate ethanol consumption at bedtime would result in memory loss for recently learned cognitive procedural and declarative tasks. The aim of Experiment 2 was to establish that the memory loss due to alcohol consumption at bedtime was due to the effect of alcohol on sleep states.

In Experiment 1, participants were asked to learn a cognitive procedural task and a declarative task in the evening. Then, either the same evening or 2 nights later, they were asked to drink ethanol (0.7g/kg). Sleep was monitored for 3 days and re-testing of the tasks was done on the eighth day after training at the same time of day. In Experiment 2, subjects were asked to learn a cognitive procedural task (Tower of Hanoi) and a motor procedural task (Pursuit Rotor) in the late afternoon. Then one group was asked to drink ethanol (0.9 g/kg) right after task acquisition (5 hours before bed), while the other was asked to drink the same dose of ethanol just prior to bedtime. Re-testing was done 8 days later at the same time of day.

Subjects in Experiment 1 were 15 college students between the ages of 19 and 24 that appeared to be in good health and were relatively naive in terms of drinking alcohol. Subjects in Experiment 2 were 13 college students in the same age range. These subjects were considered to be more experienced drinkers than subjects in Experiment 1 but were not judged to be heavy drinkers.

In Experiment 1, the alcohol ingestion resulted in memory loss for the cognitive procedural task but not the declarative task. Further, the effect was seen when alcohol ingestion occurred the same day or 2 days after the end of acquisition. In Experiment 2, alcohol ingestion at bedtime impaired memory for the cognitive procedural and motor procedural tasks. By contrast, alcohol ingestion in the afternoon, immediately after the acquisition of these two tasks, did not impair memory. There were clear changes in the nature of rapid eye movement (REM) sleep as a result of evening alcohol ingestion. In Experiment 1, the number of REMs and number of minutes of REM in the first half of the night were reduced. In Experiment 2, the reduction was in REM densities in the first half of the night.

Moderate doses of ethanol can modify the REM sleep architecture by reducing the number of REMs and/or REM densities as well as minutes of REM sleep, particularly in the first half of the night. These modifications result in memory impairment for recently learned cognitive procedural material. Alcohol may also have a subtle effect on Stage 2 sleep as well, since memory for a Stage 2 sensitive motor procedural task was also impaired.