"You know what the debate prep is? It's resting. It's lying down, going to sleep," Trump told supporters in Colorado Springs, Colo.
Trump initially resisted attacks on Clinton over her health, following her collapse after a Sept. 11 memorial event in Manhattan. 
But in recent weeks, the GOP presidential nominee has attacked Clinton's health in paid advertising, suggesting she's unfit to serve as commander in chief.  
Clinton, the Democratic nominee, pared back her campaigning recently. When asked about her schedule, staffers said she's preparing for Wednesday night's third and final presidential debate of 2016 in Las Vegas. 
Trump mocked that explanation at his Tuesday rally.
"She's doing debate prep. She did so much debate prep," he said. "Did we beat her badly in that second debate?"
The crowd roared.
"Debate prep," Trump added, giving off a sarcastic expression. "Sort of funny. She's been doing this for 30 years. Now she has to do debate prep for five days."