People who have very restless sleep are twice as likely to suffer a heart attack, research suggests.
Scientists said tossing and turning in bed at night should be seen as a warning of future ill-health.
Research suggests that various sleep disturbances are connected to ischemic heart disease and stroke.
Also know as coronary heart disease or coronary artery disease, ischemic heart disease refers to heart problems caused by narrowed heart arteries. When the arteries are narrowed, less blood and oxygen reaches the heart muscle and can ultimately lead to heart attack.
"Poor sleep is associated with cardiovascular diseases such as ischemic heart disease and stroke but the kind of sleep disturbances that are most risky is not well documented," said lead researcher Dr Nobuo Sasaki, of the Hiroshima Atomic Bomb Casualty Council, Japan. "'Poor sleep' includes too short or too long sleep, difficulty falling asleep, and difficulty maintaining sleep."
To research further which specific sleep disturbances may be linked to these two conditions, the team looked at 12,876 residents of Hiroshima, Japan, including 6,762 men and 6,114 women with an average age 68 years.
The researchers assessed participant's sleep habits using the Pittsburgh Sleep Quality Index (PSQI) questionnaire.
The PSQI assesses various aspects of sleep quality including subjective poor sleep quality, long sleep latency, short sleep duration, low sleep efficiency, difficulty in maintaining sleep, use of sleeping pills, and daytime dysfunction.
Based on participants' self-reported answers a global PSQI score is calculated, ranging from 0 to 21.
Higher scores indicated poorer sleep quality, with 'poor sleep' defined as a global PSQI score equal to or greater than 6.
The team found that 52% of patients with ischemic heart disease, 48% of stroke patients, and 37% of patients with no cardiovascular disease suffered from poor sleep.
After their analysis they also found that poor sleep quality, long sleep latency, low sleep efficiency, and use of sleeping pills were significantly associated with both ischemic heart disease and stroke.
Difficulty maintaining sleep, short sleep duration, and daytime dysfunction were only associated with ischemic heart disease.
Lead researcher Dr. Nobuo Sasaki, of the Hiroshima Atomic Bomb Casualty Council, Japan, explained that, "Difficulty maintaining sleep reflects an increase in sleep fragmentation, which refers to brief moments of waking up and causes overactivity of the sympathetic nervous system and adrenocortical axis."
Dr Sasaki concluded that, "Our results support the hypothesis that sleep deterioration may lead to cardiovascular disease. Poor sleep in patients with ischemic heart disease may be characterized by shorter sleep and brief moments of waking up."
