A few years ago, Sheryl found herself fighting a serious bout of insomnia—and, to hear her tell it, losing that battle pretty badly.

“I just couldn’t sleep. I tried everything from warm milk to no caffeine to daily cardio—you name it, if they said it would help you sleep better, I’m pretty sure I tried it,” she says. “But nothing seemed to work and it was really affecting every aspect of my life.”

Anyone who has had a poor night’s sleep knows the effects it can have. You may be fatigued, grouchy, and have difficulty focusing on even the most simple tasks. But chronic sleep deprivation, or continuous problems with sleep, can be even more debilitating. Sleep deprivation like Sheryl’s has been linked to a number of health issues—including cardiovascular disease, Alzheimer’s disease, and obesity. But what you may not know is that it is also correlated with depression and mood disorders.

According to the National Sleep Foundation, sleep disturbance is a very common symptom of depression. Some studies have even found that sleep issues can be predictive of later depressive episodes. It’s difficult to tease apart whether chronic sleep problems can lead to a clinically depressed state—or whether the underlying biology of depression gets in the way of a good night’s sleep. It’s likely that both are true. It’s well known that depression and sleep disorders share a variety of risk factors, and they can be helped by similar treatments as well.

But why might the two be so closely coupled? It may be due to new discoveries regarding sleep’s function. Recent studies suggest that sleep acts as a waste management system for the brain. The human body has the lymphatic system, a complex network of nodes filled with lymph fluid, to carry waste out of the body and maintain optimal health in your blood, skin, and organs. But the brain is not connected to that system. For many years, scientists were puzzled about why that might be. Certainly, the brain would also have waste it needed to rid itself of. But how it might do so remained an open question.

Until recently. Recent research suggests that, during sleep, cerebrospinal fluid, the fluid that surrounds the brain and spinal column, works with specialized brain cells called glia, to clear out waste. This brain-based waste removal system was named the “glymphatic” system. And, as you might imagine, when sleep is disturbed, or does not happen at all, waste and debris will accumulate in the brain, leading to neuropsychiatric disease.

This may be why clinicians so often see sleep-related problems in depressed patients. While the bulk of work on the glymphatic system has been investigating its role in Alzheimer’s disease and dementia, including the build-up of the plaques and protein tangles that interfere with healthy memory function, some scientists believe different kinds of debris may result in other problems and neuropsychiatric disorders, like depression.

Certainly, over time, Sheryl began to feel depressed.

“I was just so tired,” she says. “Not just physically but emotionally. I couldn’t concentrate. I was so cranky and, you know, just sad. It got to the point where my husband said, ‘Okay, this is a serious problem, you need to see a doctor.'”

And she was glad she did. Her physician helped her find a treatment that got her back on a sleep schedule—and, by year’s end, back to feeling like her old self.

Sleep is crucial to overall health, including brain and mood health. If you are experiencing sleep problems, don’t wait too long before seeking medical help. Your early intervention can help prevent a host of other problems, including depressed mood, later on.