It’s kind of an unwritten NFL rule that the starting quarterback should be the hardest worker on the team.

The position is so mentally challenging, opposing defenses so complex, you need to put in endless hours of mental preparation if you want to be great.

Former San Francisco 49ers QB Colin Kaepernick perhaps didn’t work hard enough in game preparation last year, a 49ers source told Sports Illustrated‘s Albert Breer.

“As one Niners employee explained it, Kaepernick wouldn’t stay late at the facility during the season like many quarterbacks routinely do, saying he’d take work home,” wrote Breer on June 22. “And there were examples where coaches saw what looked like shoddy prep surfacing in inexplicable mental errors in games.”

It’s hard to be a successful NFL QB if you aren’t putting in the requisite preparation time.

Elite quarterbacks are generally the first to arrive at the team’s training facility, and the last to leave, often putting in 14-15 hour days during the season.

Former NFL QB Peyton Manning, a two-time Super Bowl winner, once arrived at the Indianapolis Colts facility at 4 AM after a night game.

New Orleans QB Drew Brees is often spotted at the Saints training facility when teammates and coaches are gone, and janitors are sweeping around him.

According to Sports Illustrated’s Robert Klemko, Fox Sports Jay Glazer once went to the Saints facility around midnight with an assistant coach who forget something in his office, and Brees was in a classroom watching film. Glazer asked Brees what he was still doing there, and the quarterback replied, “Sometimes trying to be great is lonely.”

Seattle Seahawks quarterback Russell Wilson, known for his tireless work ethic, sleeps very little during the season.

“Preparation is separation–there is no time to sleep,” Wilson often says.

New England’s Tom Brady’s work ethic is legendary.

“Tom works really hard,” Patriots coach Bill Belichick said. “I have great respect for the way he competes off the field in terms of his preparation physically and as far as knowing our opponent and the game plans and all of that. That’s a big part of it.”

“He’s a tremendous worker,” said Patriots offensive coordinator Josh McDaniels.

So the fact that Kaepernick “wouldn’t stay late at the facility during the season like many quarterbacks routinely do” can partially explain his poor play last season.

You combine a questionable work ethic with a couple bad seasons in a row, and activism like National Anthem kneeling, and this makes him a less than desirable target in NFL free agency.