Mudawi Ibrahim Adam, a 58-year-old engineering professor at the University of Khartoum, was arrested in December by security forces as part of a crackdown on opposition leaders and activists.
"We visited him Thursday and found he had started an indefinite hunger strike since February 2," his wife Sabah Mohamed Adam said.
Ibrahim Adam, who had a heart operation in 2015 and needs to see his doctor on a regular basis, had previously been on a hunger strike between January 22 and 29.
"He told us this time his hunger strike was for an indefinite period unless the authorities bring him before a court or release him," his wife said at a press conference in the city of Omdurman.
During the first eight days of his latest hunger strike the prison doctor visited him only once, she said, adding the family has been allowed to visit him twice so far.
"Our lawyers have told us that NISS (National Intelligence and Security Service) has filed a case against him, accusing him of attempting to commit suicide."
Attempting to commit suicide is a crime under Sudanese law.
"We found he was very weak. We're really worried for him," Ibrahim Adam's wife said.
Many opposition leaders and activists were detained in December in an attempt to crush widespread protests against a government decision to raise fuel prices.
Ibrahim Adam, who has worked extensively on human rights causes in Sudan, has been arrested several times before for his work.
The government shut down a development organisation he headed in 2009.
Rights group Amnesty International said Ibrahim Adam was "at grave risk of torture and other ill-treatment" after his arrest.
His "arbitrary arrest underscores the government's desperate attempts to extinguish the last embers of dissent in the country", it said.
His wife added Ibrahim Adam's driver and a female employee had also been arrested in late December.
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