The first true day of President Donald Trump's impeachment trial was historic, the start of only the third such proceeding in the country's 230 years of existence. It also felt a bit like the first day of school.

On Tuesday morning, Senators dutifully arrived back to work from a long holiday weekend. As they walked into the US Capitol, most seemed eager to demonstrate how they had done their homework, how they believed this was a solemn undertaking, and how important it was to conduct the trial correctly and fairly.

By late afternoon though, senators were fidgeting in their seats, dozing off and passing notes.

Sen. Kevin Cramer, a North Dakota Republican, was an early arrival Tuesday morning. Cramer told CNN he needed to clean out his Senate desk to make room for the trial briefs and other documents. Thanks to impeachment trial rules, he and other senators would have no access to electronics.

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"I've been reading the briefs and stuff on iPad," Cramer said. Now, "we're going to have to rely on paper."

Later that morning, Cramer was spotted near the Senate subway carrying a heavy stack of books.

As the other senators filed into the Capitol, there was talk of who had done the assigned reading -- the briefs from the President's legal team and the House Democratic managers.

"They're both excellent. I've read both of them. They're tedious but you're going to learn a lot if you read the briefs," said Republican Sen. John Kennedy of Louisiana to a group of reporters.

"I've been reading, reading, reading, reading, reading," Sen. Dan Sullivan, the Republican from Alaska, said as he entered the Capitol, sounding exhausted even though the trial wasn't set to start for another hour.

It wasn't just the briefs, which were only released this week. Between the impeachment hearing transcripts and the documentary evidence, there was plenty of study material.

"I've been reading for three months now," Sen. Doug Jones, the Alabama Democrat, told CNN.

Other senators sounded less motivated.

"Back in college I remember I learned more from lectures than I did, generally, from reading the material," said Sen. Mike Braun, an Indiana Republican who was elected in 2018.

Due to his low seniority (97th), Braun's desk is in a back corner of the Senate chamber, where every senator is expected to sit silently while the trial is in session. But the Hoosier lawmaker insisted he won't be goofing off from the back row.

"I love to do crossword puzzles, but I can't do them. I'm just going to pay attention," Braun said. "I was always a good listener."

There was a lot to listen to once the trial started a little after 1 p.m. ET. As each team debated Democratic amendments to Majority Leader Mitch McConnell's organizing resolution to lay out the rules of the proceedings, several senators looked more engaged than others.

Sen. Mike Lee, a Republican from Utah who has worked closely with McConnell and the President on preparations for the trial, seemed enthralled with the scene. A constitutional lawyer, Lee took copious notes and strained to ensure he could see all of the exhibits being presented by Rep. Adam Schiff, D-California.

Sen. Cory Gardner, a Colorado Republican up for reelection this fall, was another prolific note-taker. Next to Gardner, Sen. Chuck Grassley, R-Iowa, glanced over repeatedly at his neighbor, giving a confused look that might have said, "Is this all going to be on the test?"

As the day went on, signs of boredom started to appear. Sen. Tom Cotton of Arkansas absent-mindedly clicked his retractable pen for about a minute, before Iowa Sen. Joni Ernst turned to look at him and he stopped. Sen. Amy Klobuchar, one of the four Democratic presidential candidates taken off the campaign trail for the trial, was chomping on a piece of gum.

While senators are not yet permitted to speak or ask questions, a few members along the back row were willing to bend the rules. Sen. Chris Coons told CNN that he and his nearest colleague, Klobuchar, passed notes "just like in class" and muttered objections to each other when they heard "arguments that didn't hold water."

At another point, South Carolina Sen. Tim Scott scribbled a note on a 3x5 notecard and handed it to Sen. Ben Sasse of Nebraska, who read it, leaned into Scott's ear, and began whispering. Scott silently laughed at whatever Sasse told him. Earlier, the two had been liberally raiding Sasse's candy stash of jawbreakers.

By 5:30 p.m., the length of the first day was wearing on the senators, with an increased number of yawns and sleepy looks. Sen. Martha McSally of Arizona had a blanket over her lap. Alaska Sen. Dan Sullivan let loose a big yawn. At one point, both Sen. Kirsten Gillibrand of New York and Sen. Jim Risch of Idaho appeared to have their eyes closed. After a moment, Gillibrand opened them abruptly and sat up straight in her chair.

With new determination, she looked forward as Florida Rep. Val Demmings, one of the Democratic impeachment managers, continued her argument before the chamber. It was nearing the end of a long day -- but only the first of many.