One of the side effects of feeling sick is the urge to hide under the covers and retreat from the world. You might just want to sleep, forget work and ignore your family and friends. Your loved ones might call you cranky, but scientists have a kinder term; they call it "sickness behavior," and say it's your body's way of helping you heal.

Researchers believe that the behaviors we exhibit when we're sick help our immune systems refocus so we're expending critical energy only to fight the invading germs that have taken over our bodies. By cocooning ourselves in an isolated environment, where we're taking it easy physically and mentally, we spend our valuable resources tackling pathogens to fight infections.

Some of the main signs of sickness behavior include sleepiness, loss of appetite, lethargy, depressed mood and less interest in social interaction.

The same chemicals in the body that trigger your immune system to fight off viruses and infections are the ones that tell your body to slow down, become a hermit and take it easy for a while.

"Those messages are so powerful they can't be ignored," Philip Chen, a rhinologist at the University of Texas Health Science Center at San Antonio, tells NPR.

Learning from mice

To take a look at how being sick affects the mind, researchers at the University of Freiburg in Germany gave mice a virus that caused a brief illness. Then they placed the mice in a container of water.

When mice are healthy, they quickly struggle to make their way out of the container. But when mice feel under the weather, they give up and float.

In the study, which was published in the journal Immunity, the researchers found that the mice spent almost twice as much time floating if they were sick, which suggested that being ill affected their behavior.

Animals and sickness behavior

It's no surprise animals also exhibit sickness behavior. When your family dog or cat stops eating, slows down and retreats to a private spot away from the bustle of the family, chances are he's feeling ill. In the wild, that's nature's way of giving an animal a chance to escape predators while an infection or other illness heals.

"The sick individual is viewed as being at a life or death juncture and its behavior is an all-out effort to overcome the disease, writes Benjamin Hart, professor emeritus of veterinary anatomy, physiology and cell biology at UC Davis in a study in the journal Neuroscience and Biobehavioral Reviews.

Calling in sick

But even though our bodies hit us over the head that we should hunker down and heal, we don't often listen. How many times have you dragged yourself into work, hacking and sneezing all over your coworkers who wished you would've stayed in bed?

Forget the fact that all your coughing and nose-blowing is just distracting; there's a good chance you're contagious. For colds, you can spread your symptoms usually a day before they start and remain contagious for up to a week. You can spread the flu a day before you even have symptoms and the typical incubation period is one to four days, but can last up to two weeks, reports MedicineNet.

So although you're trying to be a good employee, that sickness behavior trying to suck you back into the couch may also be protecting your coworkers from your germs.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

It's not crankiness, it's 'sickness behavior'
When you're sick and want to crawl into bed and stay home, that's your body trying to divert energy to helping you heal.