It's been a big week and you feel exhausted, and suddenly you find yourself crying at a nice nappy commercial. Or maybe you are struck with a cold or the coronavirus and the fact your partner used up all the milk just makes you want to weep.

You may indeed feel sad about being sick or tired, but why the tears? Why can't you hold things together?

Tears serve multiple psychological functions. Tears act as a physical indicator of our inner emotional state, occurring when we feel intense sadness or intense joy.

Inside our brains, strong emotions activate the central autonomic network.

This network is made up of two parts: the sympathetic system (which activates our fight-or-flight response when we perceive danger) and the parasympathetic nervous system, which restores the body to a state of calm.

Strong emotions activate the sympathetic part of this system, but when we cry, the parasympathetic part is activated, making us feel better.

What happens when we're stressed or tired?

We are trained from a young age to control our emotions, with socially sanctioned times to express emotion, refraining from physical displays of negative emotion.

For example, crying during a sad movie is fine, but crying at work is usually seen as less acceptable.

The prefrontal cortex, or the cool, thinking part of our brain, responds to the emotional signals released by the central autonomic network, helping us regulate the emotional response to deal with our emotions in controlled ways. The prefrontal cortex is like the main processor of your computer, managing tasks to keep the system functioning well.

Unfortunately, the more stressed and tired we are, or if we experience extended periods of physical or emotional pain, the sympathetic system remains activated.

The prefrontal cortex becomes overwhelmed, like a computer that has too many programs running all at once.

The brain becomes less able to regulate our emotions in the expected ways, resulting in visible emotional responses, such as tears or angry outbursts.

We might not even realize how overwhelmed we are until tears are running down our face after a seemingly minor incident or experience.

Some people are more likely to cry than others. Women tend to cry more than men, though the extent to which this is due to biological aspects versus expectations of society is unclear.

People who score high on the personality traits of empathy or neuroticism are more likely to cry more often.

Excessive crying can also be a physical indication of depression, as the brain is overwhelmed with emotional pain.

What's the point of tears?

Beyond psychological reasons, tears play several social roles. Even as our society might disapprove of strong expressions of emotions, tears actually help to create and sustain social bonds.

Tears can act as a cry for help, visibly showing others we are not OK and need support.

Tears often generate feelings of sympathy in others, helping us connect with them.

Tears can also occur when we feel deep sympathy for another person, crying along with them, which further strengthens social bonds.

Beyond psychological and social reasons, there are also physical reasons for tears.

For instance, when we are tired, we work hard to keep our eyes open, which dries out the eyes. Our bodies produce tears to counteract the dryness, keeping the eyes moist so we can see clearly.

Watery eyes are also common in respiratory illnesses such as cold, flu, and the coronavirus.

When we have an infection in the body, white blood cells are mobilized to fight the bug. These extra white blood cells can inflame the blood vessels in the eye, which causes the eye ducts to clog, bringing tears.

Tears are a natural part of human functioning. Especially with the pressures the past few years have brought, sometimes there's nothing better than a good cry to relieve overwhelming emotions.

But if you find yourself excessively crying, it might be helpful to talk to your doctor about possible physical or psychological causes.

--Peggy Kern, Associate professor, The University of Melbourne