This study examines the day–night pattern of sudden death from cardiac causes in individuals with obstructive sleep apnea (OSA). The researchers analyzed data from 112 Minnesota residents who had undergone polysomnography and died suddenly from cardiac causes between 1987 and 2003. They compared the rates of sudden death from cardiac causes in individuals with and without OSA across four six-hour intervals of the day. The results showed that sudden death from cardiac causes occurred more frequently in individuals with OSA between midnight and 6 a.m., compared to other times of the day. Specifically, 46% of individuals with OSA died from cardiac causes during this period, compared to 21% without OSA, 16% in the general population, and 25% expected by chance. Individuals with OSA who died from cardiac causes during this time had a significantly higher apnea–hypopnea index, which is a measure of the severity of sleep apnea. The relative risk of sudden death from cardiac causes during this time was 2.57 for individuals with OSA, compared to 0.77 for those without OSA. The study also found that the severity of OSA correlated directly with the risk of nocturnal sudden death from cardiac causes. The findings suggest that individuals with OSA have a peak in sudden death from cardiac causes during the sleeping hours, which contrasts with the nadir in the general population. The study highlights the importance of identifying and managing OSA to reduce the risk of sudden death from cardiac causes. The study was supported by grants from the National Institutes of Health and the Dana Foundation.This study examines the day–night pattern of sudden death from cardiac causes in individuals with obstructive sleep apnea (OSA). The researchers analyzed data from 112 Minnesota residents who had undergone polysomnography and died suddenly from cardiac causes between 1987 and 2003. They compared the rates of sudden death from cardiac causes in individuals with and without OSA across four six-hour intervals of the day. The results showed that sudden death from cardiac causes occurred more frequently in individuals with OSA between midnight and 6 a.m., compared to other times of the day. Specifically, 46% of individuals with OSA died from cardiac causes during this period, compared to 21% without OSA, 16% in the general population, and 25% expected by chance. Individuals with OSA who died from cardiac causes during this time had a significantly higher apnea–hypopnea index, which is a measure of the severity of sleep apnea. The relative risk of sudden death from cardiac causes during this time was 2.57 for individuals with OSA, compared to 0.77 for those without OSA. The study also found that the severity of OSA correlated directly with the risk of nocturnal sudden death from cardiac causes. The findings suggest that individuals with OSA have a peak in sudden death from cardiac causes during the sleeping hours, which contrasts with the nadir in the general population. The study highlights the importance of identifying and managing OSA to reduce the risk of sudden death from cardiac causes. The study was supported by grants from the National Institutes of Health and the Dana Foundation.