Indonesia is investigating local carrier Batik Air after both pilots were found to have fallen asleep for 28 minutes mid-flight.
The two men - who have both been temporarily suspended - fell asleep during a flight on 25 January from Sulawesi to the capital Jakarta.
One of them was reportedly tired from helping care for his newborn twins.
The Airbus A320 briefly veered off course but landed safely, with all 153 passengers and crew members unharmed.
The 32-year-old pilot had told his co-pilot to take control of the aircraft about half an hour after take off, saying he needed to rest. The 28-year-old co-pilot agreed, according to a report from the transport ministry.
But the co-pilot inadvertently fell asleep as well. According to the report, he had been helping his wife look after their one-month-old twin babies.
Jakarta air traffic control tried contacting the cockpit of the Batik Air A320 after their last recorded transmission but received no response.
That radio silence lasted 28 minutes until the lead pilot woke up and realised that his co-pilot had also fallen asleep. He also found that the aircraft had briefly veered off course.
The pilots then responded to calls from Jakarta and landed the plane safely.
Medical tests before the flight deemed that the men were fit to fly. Their blood pressure and heart rate were normal and alcohol tests came back negative.
But while the pilots appeared to be fully rested, the tests failed to determine whether the quality of their rest was good, aviation expert Alvin Lie told OceanNewsUK.
Authorities have now "strongly reprimanded" Batik Air over the incident, with Indonesia's head of air transport, M Kristi Endah Murni, saying that Batik Air should pay more attention to their crew's rest time.
Batik Air has said it "operates with adequate rest policy" and that it was "committed to implement all safety recommendations".
In 2019, the same airline was forced to have an emergency landing after the pilot fainted.
Aviation regulations in most countries require at least two pilots to be present in the cockpit of commercial airliners.
