Authored by Trombyak Ghosh.

As adults, we spend approximately one third of our lives sleeping. At least that’s the amount of sleep experts recommend. Again and again we hear the mantra about getting 8 hours of sleep per night, but do you know why?

Even though sleep is one of the body’s most basic functions and is something all people do—as well as animals with complex nervous systems—there is still a veil of mystery around it. Scientists are still not completely certain why people sleep or exactly how it evolved. However, they have come up with many hypotheses to explain why sleep is important and how it affects our brain and body.

Brain function. Without sufficient sleep, your brain cannot function properly. Lack of sleep leads to problems with concentration, performance, cognition, memory, and productivity. When you get sufficient sleep, however, research has shown that both kids and adults have better memory and problem-solving skills.

Cardiovascular risk. People who get less than 7 hours of sleep per night are at a much greater risk of cardiovascular disease (stroke, coronary heart disease) than those who get 7 to 8 hours of shuteye, based on the findings of more than a dozen studies.

Depression. Several mental health issues, including depression have been linked to inadequate sleep and sleep disorders. One example is sleep apnoea, which is associated with poor sleep as well as significantly higher rates of depression than those without this sleep problem. Overall, about 90 percent of people who are depressed also have sleep quality challenges, including inadequate sleep.

Emotional and social life. Some researchers have reported evidence that inadequate sleep reduces your ability to recognize important emotional cues from other people, including happiness and anger. This factor may make it difficult to interact socially with others.

Immune system. One of the best things you can do to fight off a cold or the flu is to get enough sleep. That’s because it’s been shown that people who sleep less than 7 hours per night are nearly three times more likely to get the common cold than those who sleep 8 hours or longer.

Inflammation. It’s been shown that inflammation plays a critical role in many serious health challenges, ranging from heart disease to asthma, arthritis, inflammatory bowel disease, and diabetes, among others. Therefore, the fact that sleep can trigger markers of inflammation and cell damage is important to know. One example is an association between poor sleep and inflammatory bowel diseases, which has been demonstrated in a number of studies, including one in the World Journal of Gastroenterology and another in Gastroenterology & Hepatology.

Physical performance. Inadequate sleep can slow you down physically. Whether you are taking a walk, going about your daily routine, or playing a game of tennis, you need sleep to perform your best. In a study of older women, for example, poor sleep was associated with greater difficulty performing daily activities, walking, and having grip strength. People who are typically active, such as those who participate in a variety of sports, also have better speed, recovery times,  People who are typically active, such as basketball players, also have better performance and recovery times when they get adequate sleep.

Type 2 diabetes risk. Do you get less than six hours of sleep per night? Then you are at increased risk for type 2 diabetes. Inadequate sleep overall has a negative impact on blood glucose levels in the general population as well.

Weight. Research has shown that inadequate sleep is associated with an 89 percent and 55 percent greater likelihood for children and adults, respectively, to be obese.

One reason for this relationship appears to involve hormones. When we don’t get enough sleep, our appetite hormones are disrupted. For example, levels of the appetite stimulant ghrelin rise while those of the appetite suppressant, leptin, decline. These responses can lead to weight gain.