"The solitude is weighing on me and I am miserable," Swiss activist Marco Weber, 28, said in a letter published by the SonntagsZeitung and Le Matin Dimanche weeklies.
He said he had been held for 24 days in isolation in the prison in the northern Russian region of Murmansk, and had no contact with the outside world besides regular visits from the Swiss consul.
"The situation is hard to take . . . the days are long," he wrote in answer to written questions submitted to him.
Weber said he had to remain huddled up in his winter jacket to try to keep warm in the unheated cell, and his feet were cold.
He said he found his daily exercise break "degrading".
"I walk around alone in a closed-off space measuring four by five metres, with concrete floors and walls and no windows, no natural light," he said, adding that the space was "dirty and humid".
But Weber said he was "sure he did the right thing" in joining the protest at a Gazprom oil platform on September 18, which resulted in 30 Greenpeace activists being arrested and charged with piracy.
Weber's letter was written before Russian prosecutors last week reduced the charges to hooliganism, which carries a lesser sentence of up to seven years in a penal colony.
Letters and notes from two British activists were also published in newspapers on Sunday, revealing marginally better conditions but a similar level of anguish.
"In many ways I'm lucky I don't have children who depend on me," wrote Kieron Bryan, a 29-year-old video-journalist, in response to questions submitted by the Sunday Times.
"But the fear of losing years of my life and the opportunity to perhaps start a family is terrifying."
Bryan said the hardest moment of his incarceration was his first night in prison where he was placed in a cell with Russian criminals.
"Once it was clear we weren't in physical danger you adapt to the regime and its severe limitations," he wrote.
"Now the difficulty is the silence and ignorance imposed by our detention.
Any shred of news or kind message that filters through the layers of bureaucracy is clung to."
He described his cell, which he shares with another man, a Russian speaker, as eight by four metres.
"I spend 23 hours a day in here with nothing but the occasional book and my thoughts," he wrote.
"We are granted an hour a day for exercise which is held in a shed about 30 metres from my cell.
"If I'm lucky I might get to shout a quick hello to an English speaker."
Bryan complained about the food — lunch was a soup and fish stew that "tastes like an ashtray full of seawater" — and admitted his disappointment when the 'chicken' he had looked forward to turned out to be inedible pigeon.
Another Briton, seasoned activist Frank Hewetson, wrote to the Independent on Sunday, offering a dry take on his predicament, styled as if he were offering a travel feature.
"The observations may be a trifle limited to 30-foot (nine-metre) wall, rolls of barbed wire, dog fence, watch tower and more barbed wire but one does get smells and some ambience of location," the 45-year-old wrote.
He said the cell, which he appears to share with two heavy smokers, measured five metres by two metres, but joked this is "somewhat larger" than some Japanese hotel rooms.