On 25 January 2024, an Airbus A320 First Officer fell asleep aware that the Captain was already sleeping en-route. Prior approval to temporarily deviate around adverse weather had been given followed by an instruction to join a destination arrival procedure which began 125 nm away. After over 200 nm of further flight and almost half an hour out of ATC contact, the intentionally-sleeping Captain woke up, took control and woke up the other pilot. ATC were then contacted for instructions on regaining the arrival route after a significant unapproved deviation from clearance and the flight was completed without further event.