When Prime Minister Lee Hsien Loong was taken ill on Sunday night during his National Day Rally speech, he suffered what doctors call vasovagal syncope.
It refers to a temporary loss of consciousness caused by a fall in blood pressure because of dehydration or standing too long.
Commonly, it is described as feeling faint. It happens when a person is dehydrated owing to not drinking enough liquids. The volume of blood in the body is reduced and not enough is pumped from the heart to the rest of the body, including the brain, said cardiologist Paul Chiam of Mount Elizabeth Hospital.
Similarly, when a person stands still for too long, blood pools in the legs and not enough goes to the brain. He would start to feel light- headed. Dr Chiam said: "We see vasovagal syncope often at parades."
The way to prevent it is to wiggle your toes and tighten the leg muscles regularly.
Vasovagal syncope has no long-term ill effects, unless it happens frequently. Even then, the biggest worry is when the person faints, falls and hits his head.
Dr Chiam said: "Once you lie down, you will recover in a few seconds as blood flows to the brain."
Could it have been a stroke?
It was the first thought that crossed the mind of Professor Tan Huay Cheem, director of the National University Heart Centre, Singapore, who had watched PM Lee live on television on Sunday.
Mr Lee had trembled and faltered, and was gripping the sides of the rostrum. He looked pale.
"I had the fright of my life. I thought he had a stroke," said Prof Tan. But he soon realised it was not even a minor stroke because "he could walk and smile" as he was supported off the stage.
The fact that PM Lee could return to finish his speech left no doubt that it was vasovagal syncope.
"People who have a stroke don't recover so quickly," said Prof Tan.
Many things can trigger a fainting spell, including exhaustion, prolonged stress, anger, and being in a hot and stuffy environment.
PM Lee has had a hectic schedule in the past weeks, with three official visits in July - to Malaysia, Mongolia and the United States - followed by events at the national and constituency levels almost daily this month.
Signs of imminent fainting include blurred vision, seeing stars or bright sparkles, dizziness and nausea, Prof Tan added.
Both doctors stressed that vasovagal syncope is temporary and has no bad after-effects. Said Prof Tan: "Many of us will feel faint or even faint some time in our lives."