The driver of a big-rig truck that crashed through a center divide on U.S. Highway 101 near Menlo Park Sunday morning likely fell asleep at the wheel, according to the California Highway Patrol.

The truck was traveling north on Highway 101 when the driver appeared to have fallen asleep and veered to the right, coming into contact with a concrete barrier on the side of the roadway at about 2:20 a.m., CHP Officer Art Montiel said.

The driver then steered the truck to the left and lost control, Montiel said.

The big-rig crashed through the center divide and the cab overturned on its side, sliding north for about 100 yards. The trailer came to rest on top of the center divide.

The driver and his son were not injured in the crash, and no other vehicle is believed to be involved in the crash, Montiel said.

Portions of Highway 101 were blocked for nearly six hours, with all lanes opening up by 9:15 a.m.

The collision remains under investigation. Anyone who might have witnessed it is asked to contact CHP Officer Rathe at (650) 369-6261.