Heart attacks and strokes are most likely to happen in the morning, research has found.
High blood pressure is a bad sign
Ambulance data shows emergency call-outs for the life-threatening conditions peak between 8am and 11am.
The British Cardiac Patients Association, which carried out the research, hopes to raise awareness of the need to monitor blood pressure.
Research shows that the majority of potential victims are unaware that high blood pressure is a major threat.
Darren Reynolds, clinical audit manager for Surrey Ambulance Service, said: "The figures here all tell the same basic story: the hours just after waking seem to be the riskiest for suspected heart attacks and strokes.
"We would also urge the public not to ignore any suspicious symptoms - like chest pain or numbness - they may perceive on waking."
Ten ambulance NHS trusts collated figures for average 999 call-out times for suspected heart attack and/or strokes during 2003.
The 'peak' time was the same in all regions. In London, there was an average of 238 call-outs per month between 8am and 11am.
Dr John Pittard, a GP and founder member of the Primary Care Cardiovascular Society, told BBC News Online that blood pressure tended to be lowest during sleep, and to surge when you get up.
"The stress of getting up, rushing to work, getting stuck in traffic can all get our stress hormones circulating," he said.
Dr Pittard said it was important that people who were on blood pressure medication ensured that they took it at the right time, so they were protected at the most vulnerable times of day.
The BCPA warned that although different factors played a part, high blood pressure was the main risk.
One in three women and two in five men suffer from high blood pressure but only 10% succeed in bringing it under control, while most are unaware of the danger.
A survey showed only 23% mentioned the condition as a risk for stroke or heart attack.
Smoking was put at the top of the list by 58% of people asked while obesity, lack of exercise, high cholesterol and over-consumption of alcohol were also
Eve Knight, service development officer for BCPA, said: "Although some people
are at greater risk of developing high blood pressure than others, it can affect anyone.
"Along with watching your weight, not smoking and eating a healthy diet, we would urge everyone to get their blood pressure checked on a regular basis and, if it is high, to take medical advice to lower it.
"This is one of the most proved ways of reducing your long-term risk of strokes and heart attacks."
The Wake-Up Call campaign is being run in conjunction with Healthy Heart UK,
a charity working to reduce coronary heart disease.