Four people died when the Metro-North train derailed while speeding through a sharp curve.
The Metro-North engineer in Sunday's deadly crash may suffer from a sleep disorder that caused him to zone out right before the train derailed.
William Rockefeller, 46, will undergo an overnight sleep evaluation to determine if he has sleep apnea, The New York Daily News has reported.
Sleep apnea is a condition that disrupts sleep. Breathing pauses or becomes shallow, causing sufferers to have a poor night's sleep and become tired during the day.
Rockefeller is currently on unpaid leave following the Bronx crash that left four passengers dead and more than 70 others injured.
The seven-car train was traveling at 82 miles per hour, nearly three times the 30-mph speed limit for the area of curved track where it crashed.
Rockefeller told investigators that he had somehow slipped into a daze. He eventually snapped out of it and applied the brakes, but it was too late to prevent the derailment.
Sources said earlier this week that he most likely experienced highway hypnosis, a condition where a person can drive a long distance with little or no recollection of the trip.
According to a source familiar with the railroad's operations, the train was equipped with an alert system that goes off when it senses that the engineer is idle, but it was not installed in the car from which Rockefeller was controlling the train.