Chances are by the time you finish reading this article you will yawn at least once. It’s OK — you’re probably sleep-deprived and have been for months.

People sleep about an hour and a half less than a century ago, and the likelihood of not getting enough ZZZs on a regular basis has increased significantly over the past three decades as the boundaries between work and home have blurred.

A third of American adults say they sleep less than the recommended seven to nine hours. The optimal amount of sleep is highly dependent on individual needs, but most people would likely be happier, healthier, and safer if they had between 60 and 90 minutes more sleep per night, according to the American Psychological Association.

To identify the 21 health problems associated with sleep deprivation, 24/7 Tempo consulted a sleep medicine specialist and a clinical psychologist specializing in insomnia treatment, and reviewed dozens of medical studies in publications such as the Journal of Neuroscience, Nature Communications, and Oxford Academic.

The fact that we need to spend a third of our day sleeping is a sign it’s vital for health. Insufficient sleep, which has been linked to diabetes, heart problems, obesity, and depression, increases mortality risk, according to Dr. Mayank Shukla, a sleep medicine specialist in New York.

Sleep occurs in several stages, each with its own role in promoting good health, according to Shukla. “All stages are important, so make sure you get them all.” The first lasts a few minutes; your eyes are closed but you’re still alert. The second is light sleep; your heart rate slows and your temperature drops. Next is deep sleep when the body repairs muscles and tissues. The last phase is REM sleep, during which you dream and form memories. These are the most common reasons for memory loss and brain shrinkage.

The body recovers during sleep because the metabolic rate is lower, Shukla noted. “When metabolism is slowed, the body goes into rest mode,” he said.

People sleep in cycles, each lasting about 90 minutes, with the rapid eye movement (REM) sleep stage getting longer throughout the night. Here are 23 simple tips for a better night’s sleep.

The first half of the night is mostly non-REM, and the second half is mostly REM, which is when you are in deep sleep and resting. That’s why you feel tired after waking up in the middle of REM sleep, even if you’ve slept enough hours, according to Joshua Tal, Ph.D, a psychologist with a specialty in sleep disorders.

Signs that you may not be getting as much sleep as you should include nodding off at at work, in front of the TV, or in other situations; mood changes, such as little things bothering you a lot more than before; and changes in energy capacity, Tal noted.

Click here to see 21 ways you could be doing damage to your body by not getting enough sleep