Viral factors?
According to some press accounts, the Captain had dinner with his family the prior night, and his daughter, 3 months old, had her very first "cold" and was up fussing all night. Possibly he had seen his family on Friday as well.
The reason that may be relevant is that he could have had a viral infection that wasn't yet full-blown, but was interfering with his performance. (Or, he took an antihistamine - a class of drugs banned in Sweden for drivers as doing more damage to driving than alcohol, but legal in the USA.)
In either case, one unresolved issue for all pilots is pre-flighting the pilot. It is very hard to tell, without an objective external test, how badly one is impaired or fatigued. I saw a video of a patient with half his brain shut down chemically, and he reported that he "just felt a little bit sleepy."
So, question to the esteemed list members. If there was a 1 minute test you could take, totally private from big-brother, that would tell you how fit or impaired you were before that first flight of the day, would you take it?
This isn't just a question for pilots. It's relevant for bus-drivers, industrial equipment operators, ATC staff, the first-responders, the military, and managers as well. It's maybe more relevant 4 hours into a flight or emergency-response as it is at the start.
Anyone can get hit with a virus, and before it becomes clinical it can still be dragging you down. Should there be some way to test fitness versus impairment? Would it work if it were secret, or would big-brother destroy it? What do people think?