Sleep is critical for maintaining good mental and physical health. Poor sleep patterns are associated with serious health problems, as well as increased stress and risk of accidents. With more than 1 in 3 Americans routinely not getting enough sleep, widespread sleep deprivation is a serious problem in the United States.

According to experts, reasons for insufficient or poor sleep can vary from physiological and psychological problems to personal habits and even bedroom conditions.

To identify the 15 reasons people have trouble sleeping, 24/7 Tempo reviewed several articles published by organizations that specialize in sleep patterns, namely the National Sleep Foundation. We also interviewed founder of NYC Sleep Doctor and clinical psychologist, Dr. Janet Kennedy.

The recommended amount of sleep for adults is seven hours or more. However, Dr. Kennedy explained the suggested amount is n

ot applicable to all, and those who need less sleep may actually hinder their sleep cycle if they try to tack on more.

“Most people need seven to about eight and a half hours of sleep a night, but the problem with focusing so much on a specific number is that people do a lot of damage to their sleep trying to achieve a number that isn’t right for them,” Dr. Kennedy said.

Conversely, some people require more sleep. Because the length of sleep each person needs varies, Dr. Kennedy suggests placing less emphasis on how many hours we sleep each night and more on the quality of sleep.

“An optimal night’s sleep is the amount of sleep a person needs to feel rested during the day so they don’t fall asleep spontaneously, feel reasonably alert and happy, able to function comfortably, and they can achieve that without assistance of substances, whether it may be alcohol or medications,” said Dr. Kennedy.

Many factors contribute to low-quality sleep, including sleep disorders, consuming stimulants like caffeine and nicotine too close to bedtime, and even the temperature of the bedroom. Here are 21 things that happen to your body when you don’t get enough sleep.

Click here to see the 14 reasons you’re not getting enough sleep