"I always feel like it's going to be a battle when the sun's up," Arroyo said.
Entering Sunday's action, Arroyo was 1-4 with a 5.17 ERA this season in five daytime starts. His sixth start in natural light was no less difficult.
Arroyo allowed five earned runs on nine hits, including two home runs by Brian Giles, in six innings as San Diego defeated Cincinnati, 10-4, before 31,927 fans Sunday afternoon at steamy Great American Ball Park.
"It's really hard for me to wake my body [for day games]," said Arroyo. "It's a tough thing, for me personally. I didn't have great stuff today. I had good command. They hit a couple of balls off the end of the bat that could've been outs. They got the big hits when they needed them."
Arroyo (5-13) struggled from start to finish on Sunday, walking one and striking out five in a 98-pitch effort.
"It would've been a much different game if Arroyo was at his best," said Reds interim manager Pete Mackanin.
Giles hit a 1-2 pitch from Arroyo into the right-field seats to give the Padres a 1-0 lead. It was Giles' first career leadoff home run.
"Too many bad pitches when we get ahead of hitters," said Mackanin. "That's what has disappointed me the whole series. We just couldn't seem to put guys away."
Khalil Greene singled home two runs in the third to make the score 3-0.
Mike Cameron, who scored from first base on Greene's single, collided with Reds catcher David Ross at home plate just ahead of Adam Dunn's throw.
Ross was tended to in front of home plate by team trainer Mark Mann. He remained in the game but was replaced by pinch-runner Javier Valentin after singling to lead off the third. Valentin stayed in the game at catcher.
Ross was diagnosed with a headache and concussion-like symptoms. He was later taken to Good Samaritan Hospital, where he underwent a CT scan, which was normal. He'll be reevaluated Monday.
"He was a little woozy when he came out," said Mackanin. "I don't know how serious it is. I'm concerned about it."
Padres starter Jake Peavy (13-5) allowed four runs, two earned, on seven hits in 6 2/3 innings to earn the win.
Giles' second home run of the game, his fifth of the series, made the score 5-0. It's the first time in his career that he's homered five times in three games.
"You have to tip your cap to him," said Mackanin of Giles. "I know he's capable of hitting home runs. The last time I saw a guy hit five in a series was Mark McGwire in St. Louis."
Norris Hopper extended his career-high hitting streak to 10 games with a first-inning single.
Jeff Keppinger collected his first career triple, leading off the fifth, and scored on Edwin Encarnacion's ground out to make the score 5-1.
Josh Hamilton, who returned from his Triple-A rehab assignment with Louisville earlier in the day, reached on an error to begin the sixth and scored moments later on Scott Hatteberg's double to make the score 5-2.
Cameron tripled and scored on Greene's double to put the Padres ahead 6-3.
San Diego established a new franchise record with nine doubles and 12 extra-base hits.
The Reds attempted to rally in the late innings.
Valentin's second home run of the season, a 381-foot solo shot with two outs in the seventh, made the score 6-4. The Reds loaded the bases with two outs against Padres reliever Heath Bell, but Brandon Phillips lined sharply out to first baseman Adrian Gonzalez to end the inning.
"We have the bases loaded and [Phillips] hits it hard and lines out," said Dunn. "That's the most frustrating part. We're so much better than we've played."
The Reds' bullpen was unable to keep it close.
Eddie Guardado, making his third appearance since being reinstated from the 60-day disabled list, allowed four earned runs on three hits in one-third of an inning.
Jeff Conine hit a pinch-hit double in the ninth in his 2,000th Major League appearance. There are just 14 active players with 2,000 appearances, including Ken Griffey Jr.
"We've been pretty inconsistent across the board," said Arroyo. "Aaron [Harang] is the only guy who's given us a chance to win every time out. That's how the year's been going."
Jeff Wallner is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.