This is a very good question but it does not have a simple answer because the blink rate in humans varies tremendously – from 2 to 50 blinks per minute depending on the circumstances. For example, if you are concentrating very hard on reading something, such as this answer, you will probably blink less frequently than if you were sitting chatting with your friends (not to be recommended in class).
Why Do We Blink?
There are three types of blink: spontaneous, reflexive, and voluntary.
Spontaneous blinks are those that we don’t notice (unless we start thinking about it) and serve to keep the eyeball clean and moist. However, it has been noted that the spontaneous blink rate is more than that needed to simply prevent the eyeball from drying out – so why do we blink more than required?
One answer is that spontaneous blinking is affected by a substance called dopamine in the brain, which affects particular nerve cells – some studies have shown that giving drugs that affect dopamine levels changes the blink rate, although how exactly this works is yet to be explained. It is also known that people with Parkinson’s disease, who have lower dopamine levels in the brain, blink less frequently.
Reflexive blinks are a response to an unexpected event, for example, if an object approaches your eye (such as when someone tries to put eye drops into your eye when you have an infection). Loud noises or any other startling event can also cause reflexive blinking. This type of blink lasts a fraction of a second longer than spontaneous blinks and is a protective mechanism to prevent damage to the eyeball from potential threats.
Finally, voluntary blinks are those which you do consciously (deliberately). These also last fractionally longer than spontaneous blinks.