In Japan, neither death row prisoners nor their families are informed in advance of when the execution will be carried out.
Makoto Teranaka, director-general of Amnesty International (AI) Japan, told Al Jazeera that the research into this report was so hard because "no information can be gained as to what is going on inside these detention centres where these executons are carried out".
The report was issued as a new centre-left government prepares to take power in Japan, following its landslide victory over the long-ruling conservatives in elections last month.
"To allow a prisoner to live for prolonged periods under the daily threat of imminent death is cruel, inhuman and degrading," James Welsh, AI's health expert and lead author of the report, said.
"The treatment imposed on condemned inmates in Japan means that they face a high risk of developing a serious mental illness while on death row.
"The treatment of prisoners on death row urgently needs to be improved to prevent inmates from developing serious mental health problems."
AI said it found that prisoners on death row were not allowed to talk to one another, and that contact with relatives, lawyers and others can be restricted to as little as five minutes at a time.
"Apart from visits to the toilet, prisoners are not allowed to move around the cell and must remain seated," the group said.
"Death row prisoners are less likely than other prisoners to have access to fresh air and light and are likely to suffer additional punishments because of behaviour that may infringe the strict rules imposed on them.
"These inhuman conditions increase a prisoner's anxiety and anguish and in many cases push prisoners over the edge and into a state of mental illness,"