A 20-year study of patients with sleeping disorders revealed that those who are more obese have severe breathing difficulty while sleeping.

The findings published in Monday's issue of the Medical Journal of Australia were based on the diagnosis of patients at a sleep clinic at John Hunter Hospital in Newcastle, NSW from 1987 to 2007.

In the 14,648 cases studied during the period, patients' weight increased every year. The median body weight for men increased 11 percent from 89 kilograms to 99 kilograms. Women's body weight increased 16 percent from 73 kilograms to 85 kilograms.

The proportion of subjects who were morbidly obese increased from three percent in 1987 to 16 percent in 2007, according to the study.

Over the same period, for every unit increase in body mass index of patients, the apnoea-hypopnea index (AHI) increased by 5.5 events per hour for men and by 2.8 events per hour for women.

AHI rates the severity of sleep apnoea or breathing disruptions during sleep and hypopnea or abnormally shallow and slow breathing as mild, moderate and severe according to the number of apnoea and hypopnea.

These findings are consistent with the premise that worsening severity in sleep-disordered breathing is primarily attributable to increasing obesity, the study stated.