Germany's Lukas Maertens won the first swimming gold of the Games in the 400m freestyle final on Saturday but the 22-year-old said he barely got any sleep before returning for the 200m freestyle heats.
Maertens was only 10th fastest, enough to advance to the semis later on Sunday.
"I got two to three hours of sleep, a short nap. Now I have to catch up on sleep and regeneration," he told reporters.
"Tonight I have to really give it all again. I was still able to give it an easier go this morning, but it wasn't quite so easy.
"We couldn't toast (the gold medal) at night, it was very late. I had to make sure that I got to bed. I'll save everything for afterwards."
Ariarne Titmus powered to the women's 400 metres freestyle gold in a star-studded field that included Canadian teenager Summer McIntosh and American great Katie Ledecky, but the Australian was back in the pool early on Sunday.
Titmus, the women's 200m freestyle world and Olympic record holder, was third-fastest in the heats, nearly half a second behind compatriot Mollie O'Callaghan who topped the time charts.
"Yeah, it was good to blow the cobwebs out. It's definitely hard to back up after last night," Titmus, 23, said.
"I tried to stay as calm as I could last night and this morning I tried to forget about what happened, really. The 400 is done now, so the 200 is now the focus.
"I definitely enjoyed the medal ceremony, the crowd and the entire atmosphere out there. But practically after that it was back to business, getting ready for today."
Britain's Matthew Richards missed out on a medal in the men's 4x100m freestyle relay and he was also involved in the 200m freestyle heats, going sixth fastest.
"Last night was a late night after that 4x100m freestyle relay," he said.
"This morning it was just about getting the job done and trying to conserve as much energy as possible. It's never easy to swim easy."
