Deadly heart attacks are more common on Mondays across Ireland, new research has indicated.
Patient data from across Ireland was examined in the study around heart attacks.
The research conducted by doctors at the Belfast Health and Social Care Trust and the Royal College of Surgeons in Ireland is set to be presented today at the British Cardiovascular Society (BCS) conference in Manchester.
They analysed the data of 10,528 patients across the island, including 7,112 in Ireland, 3,416 in Northern Ireland, who were admitted to hospital between 2013 and 2018 with the most serious type of heart attack.
This is known as an ST-segment elevation myocardial infarction (STEMI) and occurs when a major coronary artery is completely blocked.
They found a spike in rates of STEMI heart attacks at the start of the working week, with rates highest on a Monday.
There were also higher rates of STEMI than expected on a Sunday.
Previous studies suggesting that heart attacks are more likely on a Monday have highlighted an association with circadian rhythm - the body's sleep or wake cycle.
Cardiologist Dr Jack Laffan, who is based in Cork University Hospital and was involved with the research, said it is a sub-analysis of a bigger project which looks at survival following serious heart attacks in Ireland.
Speaking on RTÉ's Morning Ireland, he said: "The higher rates on Mondays has been described before and actually the same phenomenon has been found with strokes.
"Other studies have found a higher rate of heart attacks in the days following the clocks going forward in the springtime."
Dr Laffan said the exact cause is not clear but "we presume it is to do with the body circadian rhythm which is our natural sleep wake cycle".
He said the most likely time of the day to have a cardio-vascular event is in the early hours of the morning between 6am and 10am.
"This is when cortisol and other hormones in our blood rise as we wake up and these hormones also rise when we are under stress and the general hypothesis is that the finding is related to alterations in hormone levels to do with the circadian rhythm."
Hundreds of people are admitted to hospital with a STEMI each year in Northern Ireland.
It requires emergency assessment and treatment to minimise damage to the heart, and this is normally performed with emergency angioplasty - a procedure to reopen the blocked coronary artery.
Dr Laffan, who also led the research at the Belfast Health and Social Care Trust, said the cause is likely to be multifactorial.
"We've found a strong statistical correlation between the start of the working week and the incidence of STEMI," he said.
"This has been described before but remains a curiosity. The cause is likely multifactorial, however, based on what we know from previous studies, it is reasonable to presume a circadian element."
Professor Nilesh Samani, Medical Director at the British Heart Foundation (BHF), described the research as vital.
"Someone is admitted to hospital due to a life-threatening heart attack every 135 minutes in Northern Ireland, so it's vital that research continues to shed light on how and why heart attacks happen," he said.
"This study adds to evidence around the timing of particularly serious heart attacks, but we now need to unpick what it is about certain days of the week that makes them more likely. Doing so could help doctors better understand this deadly condition so we can save more lives in future."
