Six to eight seconds without flow, and the lights go out.

Consciousness runs on a few seconds of flow.

Your brain stores no oxygen or fuel, so a few seconds without fresh blood drops you, and that drop is the brain protecting itself.

Your brain keeps almost no reserve. It cannot store oxygen or fuel, so it leans on blood arriving every second. Stop that flow and consciousness goes within six to eight seconds; let the systolic pressure feeding the brain fall to about 50 to 60 mmHg at heart level, which is only 30 to 45 mmHg up at the brain when you are standing, and consciousness goes just the same.¹ A faint is the brain protecting itself: when the supply drops below what it needs, it drops you to the floor, where, lying flat, blood reaches the head again and you come round in seconds.²

Most of the time you never notice, because cerebral autoregulation holds flow nearly steady as pressure rises and falls, down to a mean pressure of around 60 mmHg. Below that floor the protection runs out and flow follows pressure down.¹ Everything that makes people faint, standing too long, a vagal surge, a heart that skips its beats, works by pushing the pressure past that line for a few seconds.

The vasovagal faint, and why it feels the same every time.

The ordinary faint is a reflex. A trigger, prolonged standing, heat, dehydration, pain, fear, the sight of blood, even a hard cough or straining on the toilet, which spikes the pressure in your chest, sets off the same overshooting reflex that slows the heart and opens the blood vessels at the same time.² Pressure drops, the brain comes up short, and down you go. It is the single most common cause of fainting, and in the young it is almost the whole story.³

That woozy, warm, greying run-up is the faint announcing itself. Lightheadedness, a flush of heat, nausea, sweating, ringing ears and vision narrowing to a tunnel are the early signs of a brain losing its supply, in the seconds before it gives out.¹ Reading that warning is the difference between sitting down safely and waking up on the floor. A faint that arrives with no warning at all is a different and more worrying thing, which the danger section covers below.

Standing can drop the pressure on its own.

Stand up and gravity pulls blood down into your legs. If the reflexes that should tighten the vessels and lift the heart rate are too slow or too weak, pressure falls and you can faint. That is orthostatic syncope, and it shades into the chronic upright-flow problems, POTS, long COVID and ME/CFS, that have their own page.² The first-line fix overlaps too: more fluid and salt if your blood pressure, heart and kidneys allow, and rising in stages rather than all at once.

Most fainting is harmless. Some of it is the heart.

A short list of signs flags the few faints that come from the heart and need a doctor.

The kind that matters is cardiac syncope, a faint caused by the heart briefly failing to pump enough, from an arrhythmia or a structural problem like a tight aortic valve or a thickened heart muscle. In the Framingham study, people whose syncope was cardiac had roughly double the death rate of those who never fainted, while ordinary vasovagal faints carried no extra risk at all.⁴ That gap is why the warning signs are worth knowing on sight.

• it struck during exertion, not after you stopped;
• it happened while you were lying down or sitting;
• your heart was racing or pounding just before, or you had chest pain or breathlessness; with chest pain or breathlessness, get emergency care now;
• there was no warning at all, and you were hurt as you dropped;
• someone in your family died suddenly and unexpectedly before old age, or you have a known heart condition.

These are the features clinical guidelines use to separate a harmless faint from one that needs a heart check, usually starting with an ECG.² A faint that comes on just after you stopped, by contrast, is usually the ordinary reflex kind, but if it came with any sign above, treat it as one that needs checking. This is information, not a diagnosis: if one of your faints fits any line above, let a clinician make the call.

Fainting is the brain pulling you down so the blood can reach it again.

Catch the warning, and you can head off the faint.

For the common faint, the prodrome is your chance. The moment the warm, grey wooziness starts, get to the floor or sit and put your head low before the faint can drop you. If you cannot lie down, squeeze: crossing your legs and tensing them, gripping one hand hard in the other, or tensing your arms raises your blood pressure enough to hold the faint off. In a randomised trial of people with recurrent reflex faints, those taught these counter-pressure moves saw their fainting fall from about half to about a third, roughly 39 percent fewer than with advice alone.⁵

Most reflex fainting is benign and tends to settle. Stay ahead of dehydration, with more fluid and salt if your blood pressure, heart and kidneys allow; learn your own triggers; and rise in stages after lying or sitting. The reassuring part is the natural history: after a proper assessment, most people’s faints become much less frequent over time.² The real hazard of a harmless faint is the fall itself, so the whole game is seeing it coming.

If you faint or feel faint mostly on standing, the chronic version is standing intolerance. If the wooziness keeps getting read as a panic attack, see when the body is the cause. If you are also tired and pale, thin blood can be behind the faintness, see anemia. If yours began after an infection, see what COVID does to the vessels. The habits that keep your pressure and supply steady are on what helps.