Your watch says you slept badly — should you still drive? Fatigue and driving explained
Updated: 5 hours ago
Charles Dawson, CEO, AutoSense
On Sunday 27 September, New Zealanders will put their clocks forward an hour for daylight saving. We know we'll lose an hour's sleep. What we may be less aware of is what happens on our roads afterwards.
New Zealand research analysing 12.6 million ACC accident claims between 2005 and 2016 found road accident rates were 16% higher on the first day after daylight saving began and 12% higher on the second.
Every September we hear the familiar advice: get enough sleep, take extra care and don't drive tired. But in 2026, there's another dimension worth considering.
We have never had more information about ourselves. Many of us wake to a smartwatch telling us how long we slept, how restful that sleep was and whether our body has recovered. Our phones count our steps and track our movement. Apps such as Life360 can tell families about speeding, hard braking, rapid acceleration and phone use behind the wheel.
Technology is increasingly telling us things about our health and behaviour that we might not otherwise have known - and sometimes things we don't particularly want to hear.
Imagine waking on the Monday after the clocks change. Your watch says you've had four or five hours' sleep. You feel okay. The kids need to get to school, you need to get to work and life needs to happen. Should you drive?
The answer isn't as simple as trusting your watch. Consumer wearables aren't breathalysers, and a single sleep score can't determine whether you're fit to drive.
But we can't necessarily give our own perception the deciding vote either.
Our AutoSense Fatigue and Sleep specialist, Kat Aubrey, points to research showing people can become progressively more impaired through repeated sleep restriction without their perception of sleepiness worsening to the same degree. A 2025 experimental driving study published in the Journal of Sleep Research[1] similarly found that people whose driving performance was particularly vulnerable to sleep loss didn't necessarily recognise the extent of that deterioration themselves.
There isn't one magic number for fatigue because individual vulnerability varies enormously. But we do know significant sleep deprivation can impair driving in ways comparable to alcohol - and there are some sobering benchmarks.
A US AAA Foundation[2] study found that compared with drivers reporting seven or more hours' sleep, those who had slept four to five hours had 4.3 times the crash rate, while less than four hours was associated with 11.5 times the crash rate. Researchers found the crash risk after four to five hours' sleep was comparable to driving at or slightly above the US legal blood-alcohol limit of 0.08% - notably higher than New Zealand's 0.05% limit for drivers aged 20 and over.
Kat considers four to five hours' sleep a serious red flag before driving, regardless of whether someone believes they feel fine.
We have legal alcohol limits, breath testing and a strong social expectation that if you've had too much to drink, you don't drive. Yet fatigue can seriously impair reaction time, attention, judgement and decision-making, and for most private motorists the final assessment remains largely subjective: Do I feel okay to drive?
In commercial transport, we're increasingly taking some of that subjectivity out of the equation. Guardian by Seeing Machines technology can detect developing drowsiness and fatigue while someone is actually driving, alerting the driver and their colleagues in real time so they can respond as the risk develops.
There's something the commercial driving world can teach us: objective information can be valuable when human judgement has limitations.
So what should we do with all the information now sitting on our wrists? Treat it as information, not a diagnosis - and look at the bigger picture. How much sleep did you actually get? How long have you been awake? What time of day are you driving? How do you feel?
Is it time to move our public conversation beyond “don't drive tired” and make “am I fit to drive?” as familiar a question as “am I safe to drive sober?”
That means giving private motorists clearer guidance about the combination of factors that should make us reconsider getting behind the wheel - including how much sleep we've had, how long we've been awake, the time of day, how we're feeling and, increasingly, what our own devices may be telling us.
It also means making it normal to change our plans when those signals tell us something we don't want to hear, whether that's delaying a journey, sharing the driving, stopping for a short nap or calling someone for a lift. Saying “I'm too tired to drive” should be seen as good judgement, not an inconvenience to push through.
Your watch shouldn't decide whether you drive. But if it tells you you've had four hours' sleep and your response is “I feel fine”, perhaps the question isn't whether you trust the watch - it's whether you should place quite so much trust in yourself.
[2] Tefft, B.C. (2016), Acute Sleep Deprivation and Risk of Motor Vehicle Crash Involvement, AAA Foundation for Traffic Safety. AAA Foundation study
