This is an alarming lapse for a number of reasons. 

According to DGCA sources, both the flight’s commander and first officer fell asleep soon after the aircraft entered the Turkish airspace. Around the same time, the flight descended from 34,000 feet to 29,000 feet without anybody manning the cockpit controls, said a senior DGCA official requesting anonymity.

Irrespective of whether the flight dropped quickly or was in a shallow descent, the fact that it was 5000ft off its assigned flight level is the scariest part, especially given the crowded airspace the aircraft was in at the time. 

But it gets even worse...

DGCA sources added that in a bid to hide the serious safety lapse, the pilots did not file the flight safety report (FSR), a mandatory exercise to document any untoward incident during the course of a flight. It even failed to report the matter to the DGCA.

So not only did BOTH pilots fall asleep, they then tried to cover it up. Which makes me think something else was going on, for two reasons; firstly, modern airliners have alarms that sound if there has not been a control input for a given period of time. Secondly, for the plane to descend like that, the autopilot must have been disengaged either manually or by an input on the control column. If both pilots were asleep and the autopilot was disengaged, this story might have an altogether more tragic outcome.