Most long haul flights have 2 complete 2-person flight crews. Generally comprised of Captain, first officer, and 2 second officers. Rotation is such, that at a

Most long haul flights have 2 complete 2-person flight crews. Generally comprised of Captain, first officer, and 2 second officers. Rotation is such, that at any one time, either the Captain or the First officer is on the flight deck with one of the two second officers. Critical parts of the flight (such as take off and landing) will have the two most senior pilots in command, but less demanding parts in cruise, one of the second officers will be backing up one of the senior crew while the other 2 crew members have mandated rest. Because these are 'hot bunks', you only need 2 available for the flight crew at any time. I assume the cabin crew have a similar rotation system.