Being sober does not automatically make you fit to drive. Tiredness can affect the concentration and control needed behind the wheel, and research comparing sleep deprivation with alcohol gives a clearer sense of how substantial that impairment can become.
The practical question is whether you are rested enough for the journey. A decision about who has been drinking needs a second check: whether the person taking the keys is also struggling to stay awake.
What the alcohol comparison actually means
In a 2000 study published in Occupational and Environmental Medicine, researchers Williamson and Feyer compared the effects of sleep deprivation and alcohol in the same group of 39 participants. After 17–19 hours without sleep, performance on some tests was equivalent to, or worse than, performance at a blood-alcohol concentration of 0.05%.
An earlier study by Drew Dawson and Kathryn Reid, published in Nature in 1997, compared sustained wakefulness with alcohol using a computer-based hand–eye coordination task. The researchers estimated that performance after 17 hours awake corresponded to impairment at 0.05% blood alcohol. After 24 hours, the equivalent was roughly 0.10%.
Those figures describe performance in particular laboratory tests. They do not establish that every tired driver has exactly the same crash risk as someone with that blood-alcohol concentration, or that everyone becomes unsafe at one fixed hour.
The findings also give no assurance that driving before 17 hours awake is safe. The experiments compared performance under specific conditions; they were not a timetable for planning how long to keep driving. Their relevance is that sleep deprivation can impair abilities people need to drive safely, even when no alcohol has been consumed.
Recognise the point when the plan must change
NZ Transport Agency Waka Kotahi's Road Code lists difficulty focusing, keeping your eyes open or holding your head up among the warning signs. Repeated yawning, wandering thoughts, memory loss, missed signs and lane drift also require attention.
Treat those symptoms as a reason to stop safely. Being close to home does not answer the question of whether you can stay alert for the remaining distance. Neither does having completed the same journey many times before.
It helps to make that decision before departure, when there is time to consider another driver or a later start. Decide where you could stop and allow room in the schedule for the journey to take longer.
If you are sharing the trip, agree that either person can call for a stop. Swapping seats only helps when the replacement driver is rested enough to continue. Discuss that before setting off, rather than leaving the decision until someone is fighting sleep.
Plan for sleep, then reassess honestly
The Road Code recommends a good night's sleep before driving, preferably eight hours, and avoiding your usual sleeping hours. Plan rest breaks about every two hours or 100km and share driving where possible. Alcohol makes fatigue worse, so avoid even small amounts.
If sleepiness develops, stop somewhere safe, as far off the road as possible. NZTA advises a 15–20-minute nap in the passenger seat, followed by at least 10 minutes before restarting, making sure you are fully awake.
That waiting period belongs in the journey plan too. Waking up is a moment to reassess how you feel before taking the wheel again, rather than an automatic signal to leave.
If you remain sleepy, do not drive. Arrange help to get home, find somewhere to sleep for longer or stay overnight. Let the arrival time change when continuing would mean driving tired.
