When you’re feeling exhausted, only one word seems to matter: sleep. But a number of sleep disorders and other problems can keep you from catching those zzz’s. To help you understand what could be keeping you from the rest you need, get familiar with the vocabulary of sleep issues.


Insomnia is a common sleep disorder that affects your ability to sleep. Insomnia can be characterized by the timing of the sleep problem:

  • Initial insomnia, when you have difficulty falling asleep
  • Middle insomnia, when you wake in the middle of the night
  • Terminal insomnia, when you wake up too early in the morning

Insomnia is either primary, meaning it is not caused by another condition, or secondary, which means it’s caused by another health condition or it’s the side effect of a medicine. Insomnia can also be categorized as acute (short-term), lasting more than one night to a few weeks, or chronic (long-term) lasting 3 or more nights a week for over a month.

Circadian rhythm

Circadian rhythms are the physical and mental changes that occur throughout the course of the day and affect whether you feel awake or sleepy. They are regulated by chemicals released in your brain in response to a stimulus, such as light. For example, you wake up in the morning because of chemicals that are released as your brain reacts to sunlight. The opposite occurs with a reduction in light, so in the evening your brain responds by making you feel drowsy.

Circadian sleep disorders

Disrupting your circadian rhythm can lead to problems with sleep. Two examples are jet lag, which can happen when you travel across time zones, and shift work sleep disorder, which affects people who work at night and sleep during the day. These disorders occur because your natural sleep-wake cycles are interrupted.

Delayed sleep phase syndrome (DSPS) is the most common type of circadian sleep disorder. People with DSPS have a tendency to stay up late at night and wake up late in the day. Delayed sleep phase syndrome can interfere with work and school because the sufferer is unable to fall asleep at regular “normal” sleep times.


Hypersomnia causes you to sleep too much or feel extremely sleepy during the day, even though you aren’t sleep deprived. It can be difficult to wake up in the morning, and you may feel confused at first. An overwhelming need to nap, even in the middle of talking or eating, is another symptom of hypersomnia. But napping may not relieve the sleepiness. Hypersomnia may be caused by a health condition; another sleep disorder, such as sleep apnea or narcolepsy; or some medicines.


Known best as sleepwalking, this is a type of sleep disorder that causes people to engage in complex activities while in a deep sleep. People who sleepwalk may stay in the house, wander outside, or even drive a car, all without waking. Sleepwalking is usually spontaneous, but can be induced by some medicines, or insufficient sleep.


Nightmares are unpleasant, even frightening, dreams that usually occur during sleep. Nightmares will cause waking, and people typically remember their nightmares.

Night terrors

Night terrors most often occur in children. Unlike nightmares, night terrors occur during non-REM slow-wave sleep. People who experience night terrors are aroused from sleep and become agitated or may cry or scream and be inconsolable. The sleep terror stops suddenly after a few minutes, followed by a return to sleep. Typically, people have no memory of the episode.

Limit-setting sleep disorder

This disorder occurs when children test the limits of bedtime by trying to postpone going to sleep. They may ask for more books to be read or more water to drink, or they may simply get out of bed in order to delay sleep. Parents should set firm limits, especially bedtime, and not give in to their child’s demands.

Insufficient sleep syndrome

Insufficient sleep syndrome is characterized by extreme tiredness and drowsiness. It is caused by sleep deprivation. It’s usually the result of habits and behaviors that prevent adequate sleep, such as regularly waking early and then staying up late because of other activities.


Bruxism is a condition in which you grind your teeth or clench your jaw during sleep. Bruxism can cause headaches and jaw discomfort and may even result in tooth fractures.