Michael McDonagh was found lying dead on the floor of the seclusion room in Unit B at 6.15am on July 10, 2010.
He had no pulse, was cold to the touch and the early stages of rigor mortis had set in.
State Pathologist Professor Marie Cassidy said he had been dead for a few hours prior to being discovered.
An inquest at the Dublin County Coroner’s Court heard Mr McDonagh, who was admitted to the Central Mental Hospital from St Patrick’s Institution in 2006, was being checked by staff every 15 minutes at the time of his death.
This entailed staff observing Mr McDonagh through a glass panel. They did not enter the room to check his vital signs.
The rules have since changed and anyone in seclusion is now under constant nursing observations.
Psychiatrist Dr Maurice Clancy said that waking a patient every 15 minutes to check their vital signs was “tantamount to torture” and was sleep deprivation, which was detrimental to the patient.
Mr McDonagh, who had suffered a head injury as a child and had paranoid schizophrenia, had become very agitated about 10.45pm the evening before his death and was shouting, slapping the walls and rolling on the floor.
Mr McDonagh had been moved to a seclusion room after he assaulted three staff members on June 30.
The seclusion was stopped for a few hours on July 7 and then recommenced.
Mr McDonagh was reviewed by Dr Clancy at midnight, when he was still distressed, and again at 12.35am when he was asleep on the floor.
Between this time and 6.15am staff observing him believed that the 21-year- old was asleep.
The alarm was raised at 6.15am by care officer Barry McLoughlin who said the decision to enter the room to check on Mr McDonagh was “instinctive and intuitive”.
The State Pathologist said she was unable to determine a cause of death, but said that he most likely had a cardiac event.
Mr McDonagh was overweight and a smoker.
One of the anti-psychotic medications he was on, clozapine, can been associated with a fast heart rate, the inquest heard.
Mr McDonagh had been reviewed by a cardiologist and there were no indications to stop the medication.
A jury returned an open verdict under the direction of the coroner Dr Kieran Geraghty.
The jury recommended the installation of a monitor system if feasible in relation to privacy issues.