Driving overnight is appealing on a long trip. Empty roads, no traffic, and arriving having saved a day of the itinerary.
The risk is substantially higher than daytime driving and the reasons are worth understanding before deciding.
What the evidence shows
Crash risk is elevated at night, and drowsy driving is a recognised contributor to serious collisions.
Studies of driver fatigue have compared impairment from sleep deprivation to impairment from alcohol, finding that extended wakefulness produces performance decrements comparable to meaningful blood alcohol concentrations.
The specific danger is that the impairment is not well perceived by the person experiencing it. Drivers consistently underestimate their own fatigue, which is why the intention to stop when tired is unreliable.
Microsleeps — brief lapses of a few seconds — occur without awareness and are sufficient to leave a road at speed.
The circadian factor
Which explains why the risk is concentrated at particular hours.
The body clock produces a strong drive toward sleep in the early hours, roughly between two and six in the morning, and a secondary dip in the early afternoon.
This operates independently of how much sleep you have had, which means a well-rested driver is still fighting the clock at four in the morning.
Which is why that window is disproportionately represented in fatigue-related incidents, and why it is the specific period to avoid rather than night driving generally.
What does not work
The strategies people rely on and that the evidence does not support.
Opening windows and playing loud music, which produce a brief arousal and no meaningful improvement in performance.
Caffeine as a sustained solution. It works for a period and does not address accumulated sleep debt, and the effect diminishes with tolerance.
And the belief that you will notice when it becomes dangerous, which is precisely the judgement that fatigue impairs.
What does
The measures with actual support.
Stopping and sleeping, which is the only thing that resolves the underlying cause.
A short nap of fifteen to twenty minutes, which produces a genuine and temporary improvement, and combining it with caffeine taken immediately before, since the caffeine takes effect as you wake.
Sharing the driving with somebody genuinely alert, and rotating before the driver is tired rather than after.
And planning the schedule so that the difficult hours are spent stopped rather than driving.
When it is genuinely reasonable
Being fair, since it is not always a poor decision.
A short overnight leg by a well-rested driver who slept properly beforehand.
Two drivers sharing genuinely, with real rest for the passenger.
Departure late in the evening arriving before the early hours, which avoids the worst window.
And routes with frequent services, where stopping is always available.
When it is not
After a full working day, which is the most common pattern and the worst one, since the driver begins already tired.
Alone, on a long leg, through the early hours.
On unfamiliar roads, particularly rural roads with wildlife, where the consequences of a lapse are higher and the demands greater.
And on any trip where the saving is a few hours rather than a day, which is a poor trade for the risk.
The alternative worth considering
Leaving very early rather than driving through.
Departing at four or five in the morning after a full night's sleep captures most of the empty-road benefit without the fatigue, since you are driving during the recovery rather than the deficit.
It also means arriving during the day, which is better for finding accommodation and for the last stretch of navigation.
Having done both, the early start is better in nearly every respect and the only thing it costs is an alarm.
The vehicle and the route
Two factors that change the risk profile.
Modern driver assistance systems including lane keeping and drowsiness detection provide some mitigation, and they are an aid rather than a substitute for being awake.
Route choice matters. Divided highways with lighting and frequent services are considerably lower risk than rural two-lane roads with wildlife.
Wildlife specifically is a night hazard on many rural routes, concentrated around dawn and dusk, and a collision at speed is serious.
Which means an overnight leg on an interstate is a different proposition from the same distance on back roads.
Where to stop
Since the decision to stop is only useful if there is somewhere.
Designated rest areas, which exist on major routes and vary in facilities and in whether overnight stopping is permitted.
Service stations, which are lit and busy and are the practical option on most routes.
Truck stops, which are open through the night and are set up for exactly this.
What to avoid is stopping on a hard shoulder, which is genuinely dangerous and is a significant cause of collisions.
Identifying stopping points along the route in advance means the decision to stop does not also require finding somewhere.