What alcohol does to the measurement
Alcohol is a central nervous system depressant, and it slows the processing and decision stages of a reaction more than the transmission stages. Simple reaction time worsens measurably; choice reaction time, which requires a decision, worsens more. Divided attention — doing two things at once, like steering and watching for hazards — degrades most of all.
Consistency goes the same way as with sleep loss: more lapses, wider spread, and an increasing number of trials that are far off your normal pace.
Why you cannot judge your own impairment
The most consistent finding in this area is the gap between measured impairment and felt impairment. People under the influence rate their own performance far closer to normal than it measures, and that misjudgement grows with the dose. It is not bravado; the faculty doing the judging is impaired along with everything else.
This is why a browser test is a poor tool for deciding whether you are fit to drive. It cannot measure divided attention, hazard recognition or judgement, and those degrade before and beyond simple reaction speed.
If there is any question about whether you are fit to drive, you are not. No test on this site or anywhere else changes that answer.
The demonstration is still worth doing
- Record a sober baseline: five trials, same device, same time of day, over several days.
- Any later comparison should be done somewhere you are staying put, with no possibility of driving afterwards.
- Look at the spread and the false starts as well as the average — impulse control shows up there first.
- Run the choice mode too. The gap between simple and choice reaction widens under alcohol more clearly than either number moves alone.
Recovery takes longer than sobriety feels
Reaction and attention measures can remain below baseline after blood alcohol has returned to zero, particularly when the drinking cost you sleep — which it usually does, since alcohol fragments sleep quality even when it shortens the time taken to fall asleep. The morning after is a combination of residual effects and sleep debt, and it shows up on a reaction test more clearly than in how you feel.