Most people wake up with the sun and go to sleep when it gets dark. The circadian rhythm, or sleep-wake cycle, manages this alternation between wakefulness and sleep by synchronizing with the Earth’s light-dark cycle. A disturbance in this biological rhythm disrupts sleep. People with a circadian rhythm disorder do not sleep at the right time or have difficulty sticking to a specific schedule. They may experience severe fatigue during periods of wakefulness or insomnia during periods of sleep.
Sleep disorders related to the circadian rhythm are caused by an occasional or ongoing disruption of sleep patterns. This disruption can result from internal factors, such as brain damage, blindness or hormonal changes, or from external factors that affect the timing and duration of sleep:
Out of sync with a normal sleep-wake cycle, affected persons may sometimes have to deprive themselves of sleep in order to meet their social or professional obligations. For example, a young person who always goes to bed late because of a circadian rhythm disorder may have to get up early for classes, reducing total sleep time. This sleep debt is often the cause of severe fatigue; a loss of concentration, attention and energy; and mood disorders.
Recent studies [1,2] suggest that circadian rhythm disruption may also be associated with mental health problems, such as depression and bipolar disorder. But the authors remain cautious: while the results show the effects of circadian rhythms on mental health, research in this area is still in its infancy
Types of circadian rhythm disorders and their symptoms
Delayed sleep phase syndrome: This disorder is more common in adolescents and young adults. Sleep patterns tend to go out of sync, meaning that falling asleep and waking up occur more than two hours after the normal cycle (e.g., 3 a.m. and 10 a.m.). The person may have a normal quality and quantity of sleep, but has difficulty falling asleep and waking up at times that are compatible with daily obligations.
Advanced sleep phase syndrome: This disorder is more common in the elderly . Falling asleep and waking up occur much earlier (at least two hours, often more) than the normal schedule. Although they may have a normal amount and quality of sleep, people with this disorder have difficulty staying awake early in the evening (6 p.m. to 9 p.m.) and wake up early (2 a.m. to 5 a.m.).
Irregular sleep-wake syndrome: This disorder is characterized by variable sleep periods. Sleep is fragmented into short periods throughout the day and night. Overall, persons may get a normal amount of sleep, but the fragmented nature of the sleep can be disruptive and affect their social life. It more often affects the elderly, especially in nursing homes where they have little exposure to time cues such as natural light, daily activities and structured schedules.
Free-running syndrome: This disorder, also known as hypernycthemeral syndrome, causes falling asleep and waking up to be continuously and progressively out of sync with the normal cycle by about 15 minutes. The sleep-wake pattern is not tied to the 24-hour cycle, but to a slightly longer period. This means that affected individuals fall asleep and wake up a little later each day. This disorder is more common among blind people because daylight usually helps regulate the sleep-wake cycle.
Shift work sleep disorder: This disorder mainly affects people who work nights or variable hours. Because their work hours do not always match their circadian rhythm, they may experience an irresistible urge to sleep on the job and an inability to sleep when needed. When they do sleep, the quality of their sleep is often poor, leading to fatigue, exhaustion, headaches, concentration problems and irritability. It is estimated that at least a quarter of workers with non-standard hours suffer from this disorder. 
Jet lag: Jet lag occurs when a person crosses more than three time zones. Because the person’s internal clock is still based on the original time zone, he or she may experience a strong urge to sleep during the day and an undesirable level of alertness at bedtime. This disorder is temporary. It is estimated that the body needs one day to adjust to each time zone crossed. Jet lag can affect anyone who travels by plane, but the symptoms can be more severe and last longer when a person travels eastward and in older people in general.
Often confused with other sleep disorders, such as insomnia, circadian rhythm disorders can be difficult to diagnose and often require a consultation with a sleep specialist. To identify the problem, it may be useful for affected individuals to keep a diary for two or three weeks to record their waking and sleep times and describe their symptoms as well as their impact on daily life.
Actigraphy can also be performed using a sensor (usually worn on the wrist) that records body movements for a few days. This test helps health care professionals detect a disturbance in the circadian rhythm of sleep, since it allows them to analyze the sleep-wake cycle and its shifting phases as well as evaluate the quality and quantity of sleep. Other tools, such as melatonin measurement by immunoassay , can also be used depending on the type of disorder. 
Treatments for circadian rhythm disorders depend on the type of disorder and its severity. Most treatment plans include a combination of approaches, the most common of which are the following:
Lifestyle changes: This approach aims to improve sleep by changing certain lifestyle habits (e.g., exposure to daylight, schedule of a daily routine or planning naps) and developing good sleep hygiene.
Light therapy: Exposure to bright light (i.e., 2000 to 9500 lux), the duration and timing of which varies depending on the disorder, helps synchronize the circadian clock with the Earth’s light-dark cycle. For example, shift workers should be exposed to it before leaving for work, while the wearing of sunglasses on their way home reduces light exposure and may promote sleep upon their return.
Medication: Taking melatonin, a hormone that helps regulate the circadian rhythm, may be useful in promoting sleep. Substances promoting wakefulness (e.g., caffeine) and sleeping pills can also be used for a short period of time to adjust the sleep-wake cycle to the desired schedule.
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Antúnez, J.M. “Circadian typology is related to emotion regulation, metacognitive beliefs and assertiveness in healthy adults,” PLoS ONE, Vol. 15, no. 3, e0230169, March 13, 2020. DOI: 10.1371/journal.pone.0230169
Harry Pantazopoulos, Jason T. Wiseman, Matej Markota, Lucy Ehrenfeld, Sabina Berretta. “Decreased Numbers of Somatostatin-Expressing Neurons in the Amygdala of Subjects With Bipolar Disorder or Schizophrenia: Relationship to Circadian Rhythms,” Biological Psychiatry, 2016; DOI: 10.1016/j.biopsych.2016.04.006
Pallesen, Stale, Bjorn Bjorvatn, Siri Waage, Anette Harris and Dominic Sagoe. “Prevalence of Shift Work Disorder: A Systematic Review and Meta-Analysis,” Frontiers in Psychology, March 23, 2021. DOI: 10.3389/fpsyg.2021.638252
Taillard, Jacques and Mullens, Eric. “Les outils validés pour le diagnostic des troubles du rythme circadien veille-sommeil (TRCVS) chez les adultes et enfants,” La Presse Médicale, Vol. 47, no. 11-12 (1st part), pages 977-981, 2018. DOI: 10.1016/j.lpm.2018.10.017