Alcohol continues to affect sleepiness related driving impairment, when breath alcohol levels have fallen to near-zero.

Epidemiological findings point to very low blood alcohol levels heightening the risk of sleep-related fatal road crashes. This was further assessed using a full sized interactive car simulator. Twenty, sleep restricted, healthy young men underwent a 2 h simulated afternoon monotonous drive, having previously consumed nil alcohol or 3 units >90 min previously, and having near-zero breath alcohol (BrACs) at the start of the drive. In a repeated measures, double-blind, balanced design, driving performance, subjective sleepiness and EEG were monitored throughout. Compared with nil alcohol, the alcohol condition initially increased sleepiness-related driving impairment. However, this was not mirrored by subjective sleepiness or EEG. An unexpected reversal (i.e. improvement) in driving impairment occurred with the alcohol group, in the second hour of the drive. This was supported by a trend for improved subjective alertness. Alcohol continued to interact with sleepiness-related driving impairment after BrACs had reached zero. However, a lack of subjective perception of increased sleepiness, at this time, further points to the dangerous combination of even modest alcohol intake and sleepiness, and confirms the road crash findings. BrACs are a poor guide to driver impairment.