People come from all walks of life, and this means it’s sometimes challenging to identify our commonalities. But here’s something many of us share: We’re not getting enough sleep. Millions of Americans suffer from something called sleep deprivation, a condition in which a person doesn’t obtain enough sleep to feel well rested and function at their best.

According to a health survey from the Centers for Disease Control and Prevention, anywhere between seven and 19 percent of American adults report feeling sleep deprived every day. That means as many as 61 million people are zombie-walking around the United States at any given time.

This is bad news, because adequate sleep is critical for sustaining our physical health, mental well-being, and overall quality of life.

The symptoms of sleep deprivation often serve as the first clue that something is amiss with a person’s sleep habits. These symptoms may vary in intensity depending on whether a person’s sleep deprivation is short-term or chronic. At any given time, symptoms may manifest in one or several of the following ways:

• Daytime sleepiness and fatigue
• Difficulty concentrating
• Impaired memory
• Diminished coordination and physical strength
• Impaired mood, irritability, and/or frequent mood swings
• Impaired immunity and a reduced capacity for fighting infections
• Reduced motivation
• Symptoms of anxiety and/or depression
• Poorer decision-making capabilities
• Increased errors on the job
• Increased food cravings
• Decreased sex drive
• Falling asleep at inopportune times
• Impulsive behavior

These symptoms don’t just arise in a vacuum. They result when a variety of potential contributing factors deprive a person of sleep. We’ll explore the possible causes of sleep deprivation in the following section.

After assessing the data, researchers estimated inefficient sleep was costing Australia approximately $17.88 billion a year, including sleep disorder health costs ($160 million), productivity losses during the work day ($12.19 billion), and even premature death from both accidents and illness ($0.61 billion). They also projected a total $27.33 billion cost for the country’s general reduced well-being as a result of the collective sleep debt.

“We are in the midst of a worldwide epidemic of inadequate sleep, some from clinical sleep disorders, some through pressure from competing work, social and family activities, and some from failure to give sleep sufficient priority through choice or ignorance,” the authors said in a statement. “Apart from its impact on well-being, this problem comes at a huge economic cost through its destructive effects on health, safety, and productivity.”

Researchers recently conducted a study that found people who naturally wake up early — aka early birds — are less at risk for developing depression than those who stay up late. This is the largest study that has looked at the connection between sleep-wake preference and mood disorder.

The study, which was recently published in the Journal of Psychiatric Research, was conducted by researchers at the University of Colorado Boulder and the Channing Division of Network Medicine at Brigham and Women’s Hospital in Boston. The participants were 32,000 female nurses with an average age of 52. At the beginning of the study (which was conducted over four years), each participant was asked what their natural sleep and wake time preference was, which is scientifically known as your chronotype.

What they found was that sleep-deprived participants kept the approaching person further away, generally between 18-60% further back than they did after having a good night’s rest. This suggested to researchers that a lack of quality slumber was triggering feelings of social anxiety and discomfort among the group.

And brain scans backed this up, with fMRI readings indicating that, while watching the videos, sleep-deprived subjects experienced a flurry of activity in the neural circuit that’s activated when humans perceive a threat from another human.