Seeing stars is usually due to a lack of blood reaching the brain, and thus a lack of oxygen. Most commonly this happens after standing up quickly or straightening after bending over. You don't see it all the time because the arteries serving the brain reflexively dilate to maintain pressure, but those reflexes may be disturbed by lack of sleep or food, a hangover, or an infection. Physical trauma may also cause the reflex to malfunction, which is why stars float around Sylvester the Cat's head when he gets bonked. Rarely, seeing stars can be caused by too much blood reaching the brain. This doesn't happen much outside of the laboratory or a jet fighter, as the arteries have a very strong constriction reflex to keep it in check.
The stars you see are actually neurons in your visual cortex misfiring, a hallucination at the lowest possible level. When the neurons' oxygenation changes drastically, their membrane potential also changes. Ordinarily this wouldn't matter, because all of the surrounding neurons' potentials would change at the same time. When it happens due to standing quickly or taking a few G's of acceleration, the change happens so fast that the neurons closest to capillaries change well before the surrounding neurons. This causes them to fire spontaneously which your brain interprets as vision; you see stars.
Another possible cause of seeing stars is posterior vitreous detachment, which mostly affects people over age 50. PVD happens when the eye's vitreous shrinks and collapses, detaching it from the inner surface of the eye. This causes all manners of floaters and distortions to be visible, including ones that look just like the stars mentioned above. PVD can result in retinal breakage or detachment and thus cause blindness, so if you notice a sudden onset of seeing stars and other visual phenomena you should probably go see your doctor.
Yeah, it is very common when you see stars after bending over. As we all know that when your posture changes, your heart reactively changes its output; this makes a person hypoxia, which will make you see the stars around your head. Of course, aparting from seeing stars, after bending over; you also will feel darkness and confusion. Except the changes of the output of the heart, there is also some other reasons why we can see stars when we bended over, such as the blood rushes. That is to say, when you bend over, the blood will rushed to your head and when you come back up quickly, the blood will rush back down relatively; even they will get back from the opposite direction they came. When you find that you can see starship when you bend over, just relax because this cannot mean there is something wrong with you and your eyes. LOL.
See stars or some light spots when bending over may be cased by getting orthostatic hypotension, which is a condition that occurs with a sudden drop in blood pressure. There are many reasons causes orthostatic hypotension as follows:
1. Prolonged bed rest, over rest can cause orthostatic hypotension;
3. Alcohol. When you're drunk, you also may see stars or some light spots when bend over.
4. Age problem, it commonly occurs in older people.
It's not a serious problem unless it is accompanied by loss of consciousness, which need call for medical assistance immediately.
When you blend over, your blood may be circulated in the reversed way. The rushed blood circulation will cause your eyes to see things with light spots. However, you'd better go to check on your eyes to see whether they get the infection or inflammation because of the invisible bacterium. Then it is, you could accept the treatment to make the eyes become healthy.