About 2 million Japanese are believed to suffer sleep apnea, a disorder in which sleep is repeatedly interrupted due to obstructed breathing, an expert said Friday.

In response to an incident in which the driver of a bullet train nodded off at the controls last month, the transport ministry gathered experts on the syndrome to discuss measures to prevent accidents triggered by sleep disorders.

The driver was diagnosed with sleep apnea, which causes drowsiness during the day. Sleep apnea is also the suspected cause of a freight-train driver's slumber on the JR Sanyo Line in Hiroshima Prefecture on Feb. 16.