Two crew, autopilot, good weather, own bed at night, "office hours", long sectors, quiet airspace, servicable aircraft, scheduled flying. I can take quite a lot of this without being tired.
Single pilot IFR in bad weather, short sectors, congested airspace, aircraft snags, back of the clock operations, minimum rest periods, frequent changes between earlies and lates, changing accommodation, uncertain roster.
I've had both and I certainly know which was more tiring
