Paul Whelan described his treatment in Russian prisons in his first interview since returning home as part of a prisoner swap.

Speaking to CBS News' Face The Nation on Sunday, the former Marine said he was tortured during his five years in Russian detention.

“The Russian government wanted to put pressure on the United States by treating me badly. Sleep deprivation is considered torture,” Whelan told host Margaret Brennan.

“What the Russians did in Lefortovo Prison – that’s the FSB prison – they kept a light on 24 hours in my cell. So, sleeping was very difficult. At the labor camp for 4 years, they would come every two hours to my bed at night and they would wake me up. They’d shine a light in my face and take a picture. Every night, every two hours, I was woken up. Getting off that sleep pattern has been very, very difficult,” he said.

Whelan, who arrested in 2018 after attending a wedding in Moscow and convicted of espionage, maintained that he’d been “set up,” and added that he would further explain the complex situation in a book.

The former BorgWarner director of global security and investigations also touched on his feeling of being “left out to dry” while seeing fellow American who had been jailed there for shorter periods of time – such as Brittney Griner – released back to the US in prisoner swaps.

During his long stay as a captive of Russia, Whelan revealed that news of the Russian invasion of Ukraine was at first difficult to surmise.

“Information came in very slowly. When the Russian government started taking prisoners from our camps to go to the war, then we knew they were really in trouble,” he said.

Whelan described the state-funded Wagner Group offering prisoners the opportunity to leave captivity if they joined the battle, but suggested that in Ukraine, too, they suffered.

“From my camp 450 went. I knew all of them. Some are dead. Some have arms and legs missing. They’ve all got some sort of PTSD. They were used on the front lines to walk through minefields. They were used as cannon fodder.”