Circadian Rhythm Sleep-Wake Disorders (Part 2): The Different Types of Body Clock Disorders
This is Part 2 of a 3-part series on circadian rhythm sleep-wake disorders. You can find Part 1 here and part 3 here.
In Part 1 of this series, we looked at how your body’s internal clock, known as the circadian rhythm, helps regulate when you feel sleepy and when you feel alert. In this article, we will explore a few specific circadian rhythm sleep-wake disorders.
Not all circadian rhythm sleep-wake disorders are the same. Some people naturally fall asleep much later than others, while some become sleepy unusually early. Others struggle because their work schedule repeatedly conflicts with their body’s natural rhythm.
Understanding the different types of circadian rhythm sleep-wake disorders can help explain why sleep problems do not always fit the pattern of insomnia.
Delayed Sleep-Wake Phase Disorder
Delayed Sleep-Wake Phase Disorder (DSWPD) is the most common circadian rhythm sleep-wake disorder.
People with this condition naturally become sleepy much later than most people—often not until 2 a.m., 3 a.m., or even later. As a result, they struggle to wake up in time for work, school, or other morning commitments.
Importantly, when they are able to follow their preferred schedule—for example, sleeping from 3 a.m. until 11 a.m.—they often sleep well and obtain enough sleep.
Common symptoms
People with delayed sleep-wake phase disorder may experience:
- Difficulty falling asleep before very late at night
- Extreme difficulty waking in the morning
- Feeling alert late in the evening
- Sleeping normally on weekends or holidays when they can choose their own schedule
- Excessive daytime sleepiness caused by having to wake earlier than their body clock prefers
This condition often begins during adolescence, a time when many young people’s body clocks naturally shift later. While many teenagers gradually adjust as they get older, some continue to experience symptoms well into adulthood.
Delayed sleep-wake phase disorder is sometimes mistaken for laziness, poor motivation, or simply “bad habits.” In reality, it reflects a mismatch between the body’s internal timing and external expectations.
Advanced Sleep-Wake Phase Disorder
Advanced Sleep-Wake Phase Disorder (ASWPD) is essentially the opposite problem.
People naturally become sleepy unusually early in the evening and wake very early in the morning, often several hours before they would like to.
For example, someone may feel ready for bed at 7 p.m. and wake spontaneously at 3 or 4 a.m.
Common symptoms
- Feeling sleepy early in the evening
- Waking very early without an alarm
- Difficulty staying awake during evening activities
- Feeling alert very early in the morning
Although sleep quality is often normal, the timing can interfere with work, family life, and social activities.
This pattern is more common in older adults, although it can occur at any age.
Shift Work Disorder
Modern society depends on healthcare workers, emergency services, security staff, factory workers, transport workers, and many others who work outside normal daytime hours.
Unfortunately, the human body is generally designed to be awake during daylight and asleep at night.
Shift Work Disorder occurs when work schedules repeatedly conflict with the body’s natural circadian rhythm.
Common symptoms
People may experience:
- Difficulty sleeping during the day after night shifts
- Excessive sleepiness while working
- Fatigue
- Poor concentration
- Reduced performance
- Mood changes
- Increased risk of workplace or driving accidents due to sleepiness
Not everyone who works shifts develops this disorder. Some people adapt relatively well, while others remain persistently affected despite making every effort to improve their sleep.
Non-24-Hour Sleep-Wake Rhythm Disorder
Most people’s internal body clock resets itself every day using environmental cues, especially exposure to light.
In Non-24-Hour Sleep-Wake Rhythm Disorder, this daily resetting does not occur properly. Instead, the person’s sleep time gradually shifts later each day.
Someone may fall asleep at midnight one week, 2 a.m. a few days later, and eventually during daylight hours before the cycle slowly repeats.
This condition occurs most commonly in people who are totally blind because light cannot reach the brain’s biological clock to keep it synchronised with the 24-hour day.
It is much less common in people with normal vision.
Irregular Sleep-Wake Rhythm Disorder
Rather than having one long period of sleep at night, people with Irregular Sleep-Wake Rhythm Disorder sleep in several shorter periods spread throughout the day and night.
They may:
- Nap frequently
- Wake repeatedly overnight
- Never feel that they have a regular sleep schedule
- Have no predictable bedtime
This condition is more often seen in people with certain neurological disorders, brain injuries, or dementia, although other causes may also contribute.
Are these conditions caused by poor sleep habits?
Not necessarily.
While irregular routines, frequent late nights, or excessive evening screen use can certainly affect sleep timing, circadian rhythm sleep-wake disorders are generally more complex than simply having “bad habits.”
Factors that may contribute include:
- Genetics
- Age
- Natural differences in body clock timing
- Shift work
- Exposure to light at the wrong times
- Blindness
- Certain neurological conditions
Good sleep habits remain important, but they may not completely correct an underlying circadian rhythm disorder.
Could something else be causing my symptoms?
Yes. Many different conditions can affect sleep, including:
- Insomnia
- Obstructive sleep apnoea
- Restless legs syndrome
- Narcolepsy
- Depression
- Anxiety disorders
- Bipolar disorder
- ADHD
- Medical conditions
- Certain medications or substances
Because the symptoms can overlap, it is important not to assume that a particular diagnosis applies simply because it sounds familiar.
A healthcare professional can help determine whether a circadian rhythm disorder is present, whether another condition better explains the symptoms, or whether more than one condition is contributing.
Looking ahead
This is Part 2 of a 3-part series on circadian rhythm sleep-wake disorders. You can find Part 1 here and part 3 here.
In Part 3 of this series, we’ll explore how circadian rhythm sleep-wake disorders are diagnosed and treated. We’ll discuss sleep diaries, light therapy, melatonin, lifestyle strategies, and why treatment often depends on correctly identifying the specific type of circadian rhythm disorder.
This article is intended for general educational purposes and should not replace individual medical advice. If you have ongoing sleep difficulties, excessive daytime sleepiness, or concerns about your sleep schedule, consult an appropriately qualified healthcare professional for an assessment.
