It's sometimes called highway hypnosis or white-line fever, and it's familiar to anyone who has driven long distances along a monotonous route.
Drivers are lulled into a semi-trance state and reach their destination with little or no memory of parts of the trip.
But what if it happened to an engineer at the controls of a speeding passenger train?
A man driving a Metro-North Railroad commuter train that went off the rails Sunday in New York, killing four passengers, experienced a momentary loss of awareness as he zoomed down the tracks, according to his lawyer and union representative, who called the episode a "nod," a "daze" or highway hypnosis.
Their accounts raised questions about just how widespread the problem is in the transportation industry and what can be done to combat it.
At the time of the crash, the train was going 82 mph (132km/h) into a sharp turn where the speed limit drops to 30 mph (48km/h). That's when the engineer says he snapped out of it and hit the brakes, but it was too late. The train hurtled off the tracks, leaving a chain of twisted cars just inches from a river in the Bronx.
While the term highway hypnosis has been around for decades, there's no technical definition of it and scant specific medical study of it, although multiple studies have found that long driving times on straight roads can cause people to lose focus.
Some experts equate highway hypnosis with a sort of autopilot state - performing a task, usually competently, without awareness of it. Sleep experts say the daze could really be a doze, especially if a driver has undiagnosed sleep problems.
Whatever it is, nearly every bus or train driver has experienced the feeling of being momentarily unaware while driving long hours, said Larry Hanley, president of the Amalgamated Transit Union.
Hanley, who spent eight years driving a bus in New York, recalled spending a week on the midnight-to-8am shift and sometimes stopping to pick up passengers who weren't there.
"You find yourself stopping, and you open the doors, and all you see is a mailbox," he said, adding that fatigue and work schedule changes play a role.
The National Transportation Safety Board, which has yet to determine the cause of the crash, concluded talking with the engineer Tuesday. Investigators continued interviewing the train's other crew members. Investigators have said the engineer, William Rockefeller, had enough time off for a full night's rest before the crash, but they were looking at his activities in the previous days.
Highway hypnosis doesn't show up often in medical literature, but numerous researchers have looked at the effect that monotonous driving can have on alertness and reaction time.
In one early paper on the phenomenon, published in 1962, retired Rutgers University psychologist Griffith Wynne Williams wrote that the modern superhighway's smooth, uninterrupted stretches of concrete could put people in a daze.
"Driving under these conditions makes little demand on the driver's orientation to reality," he wrote. "The distracting stimuli are few."
It's the "Where did those 10 miles go?" sensation of realizing you've been driving apparently without paying attention to the road or yourself, said Stephen J. Morse, a professor of law and psychiatry at the University of Pennsylvania.
Many sleep experts see highway hypnosis as micro-sleep, a phenomenon often attributed to fatigue or sleep deprivation.
Most people don't even realize when they've been micro-sleeping - for example, "resting their eyes" for a few seconds, said Dr. James Maas, a sleep expert and retired Cornell University psychology professor.
"Many of those times you were asleep. You're just not going to remember it," he said.
Transportation safety advocates also have long been concerned about fatigue in all modes of transportation.
In 2008, the operator of a transit train was killed after she fell into a micro-sleep and collided with another train in Newton, Mass. Fatigue also was a factor when two trains collided in Red Oak, Iowa, in 2011, killing two crew members.
A survey of transportation workers last year by the National Sleep Foundation found 26 percent of train operators said sleepiness affected their job performance at least once a week, compared with only 17 percent of non-transportation workers. About 18 percent of train operators reported having a "near miss" at work because of fatigue, and 44 percent of train operators said their work schedule did not allow enough time for sleep.
Rockefeller's schedule, which had recently switched from the afternoon shift to the day shift, could be a cause for concern about fatigue, said Patrick Sherry, executive director of the National Center for Intermodal Transportation at the University of Denver, which studies national transportation issues.
"Did he make an appropriate transition from his previous shift to this new shift?" Sherry said.
How long that transition takes is highly individual - think jet lag, which levels some people while others adjust easily, said Dr. Clete Kushida, a neurologist and sleep specialist at Stanford University Medical Center.
Federal investigators would not comment on Rockefeller's level of alertness. The NTSB had found no problems with the brakes or rail signals. Alcohol tests on crew members were negative, and investigators are awaiting the results of drug tests.
The NTSB has issued more than 200 recommendations addressing fatigue, including scheduling problems that disrupt sleep patterns, Chairman Deborah Hersman said Wednesday in a telephone interview.
Hersman said positive train control technology, which can slow or stop a train that's speeding or otherwise not being operated correctly, might have forestalled the derailment. Railroads are facing a congressional deadline to install such systems by December 2015.
"This is the type of accident that positive train control is designed to prevent," Hersman said.
As for how to avoid micro-sleeping, a 10- to 20-minute nap or a cup of coffee can help in a pinch, suggested Dr. Clete Kushida, a neurologist and sleep specialist at Stanford University Medical Center.
But experts agree there's no substitute for getting good sleep.
Truck driver Alex Gordon agrees. He drives for no more than 10 hours at a time and makes sure to get enough sleep, and he says he's never experienced highway hypnosis.
"I drive 10 hours, sleep 11," the Miami-based Gordon said Wednesday during a break at a truck stop in Kearny, N.J. "You just can't" put people in danger, he said.
In case of an engineer becoming incapacitated, the train's front car was equipped with a dead man's pedal, which must be depressed or the train will automatically slow down.
Trains also can have alarms, sometimes called alerters, which sound if the operators' controls haven't been moved within a certain timeframe. If an engineer does not respond, often by pressing a button, brakes automatically operate. But the train that derailed didn't have such a system, a Metro-North spokeswoman said.
Rockefeller, 46, has worked for the railroad for 15 years and has been an engineer for 10.
Crews are rebuilding the damaged track where Rockefeller's train crashed. One of three tracks on the affected line reopened Wednesday, and commuters said they were grateful service was restored fairly quickly.
"We don't get to complain," said Elite Rubin, who does marketing for an accounting firm. "We weren't on that train where people died."