Circadian rhythm sleep disorders (CRSD) are a family of sleep disorders affecting, among other things, the timing of sleep. People with circadian rhythm sleep disorders are unable to sleep and wake at the times required for normal work, school, and social needs. They are generally able to get enough sleep if allowed to sleep and wake at the times dictated by their body clocks. Unless they also have another sleep disorder, the quality of their sleep is usually normal.
Humans, like most animals and plants, have biological rhythms, known as circadian rhythms, which are controlled by a biological clock and work on a daily time scale. These affect body temperature, alertness, appetite, hormone secretion etc. as well as sleep timing. Due to the circadian clock, sleepiness does not continuously increase as time passes. A person's desire and ability to fall asleep is influenced by both the length of time since the person woke from an adequate sleep, and by internal circadian rhythms. Thus, the body is ready for sleep and for wakefulness at different times of the day.
Sleep researcher Yaron Dagan states that "[t]hese disorders can lead to harmful psychological and functional difficulties and are often misdiagnosed and incorrectly treated due to the fact that doctors are unaware of their existence."
Four of them are intrinsic (from Latin intrinsecus, on the inside, inwardly), "built-in":
Delayed sleep phase disorder (DSPD), aka delayed sleep phase syndrome (DSPS), characterized by a much later than normal timing of sleep onset and offset and a period of peak alertness in the middle of the night.
Advanced sleep phase disorder (ASPD), aka advanced sleep phase syndrome (ASPS), characterized by difficulty staying awake in the evening and difficulty staying asleep in the morning.
Non-24-hour sleep–wake disorder (non-24), in which the affected individual's sleep occurs later and later each day, with the period of peak alertness also continuously moving around the clock from day to day.
Irregular sleep–wake rhythm, which presents as sleeping at very irregular times, and usually more than twice per day (waking frequently during the night and taking naps during the day) but with total time asleep typical for the person's age.
Normal circadian rhythms
Typical features of a normal cicadian rhythm, including a bedtime of 22:00.
Among people with healthy circadian clocks, there is a continuum of chronotypes from "larks", "morning people", who prefer to sleep and wake early, to "owls", "evening people" or "night people", who prefer to sleep and wake at late times. Whether they are larks or owls, people with normal circadian systems:
can wake in time for what they need to do in the morning, and fall asleep at night in time to get enough sleep before having to get up.
can sleep and wake up at the same time every day, if they want to.
will, after starting a new routine that requires their getting up earlier than usual, start to fall asleep at night earlier within a few days. For example, someone used to sleeping at 1 a.m. and waking up at 9 a.m. begins a new job on a Monday, and must get up at 6 a.m. to get ready for work. By the following Friday, the person has begun to fall asleep at around 10 p.m., and can wake up at 6 a.m. feeling well-rested. This adaptation to earlier sleep/wake times is known as "advancing the sleep phase." Healthy people can advance their sleep phase by about one hour each day.
Researchers have placed volunteers in caves or special apartments for several weeks without clocks or other time cues. Without time cues, the volunteers tended to go to bed an hour later and to get up about an hour later each day. These experiments appeared to demonstrate that the "free-running" circadian rhythm in humans was about 25 hours long. However, these volunteers were allowed to control artificial lighting and the light in the evening caused a phase delay. More recent research shows that adults of all ages free-run at an average of 24 hours and 11 minutes. To maintain a 24-hour day/night cycle, the biological clock needs regular environmental time cues or Zeitgebers, e.g., sunrise, sunset, and daily routine. Time cues keep the normal human circadian clock aligned with the rest of the world.
Circadian rhythm abnormalities
Non-24-hour sleep–wake disorder and other persistent circadian rhythm sleep disorders are believed to be caused by an inadequate ability to reset the sleep/wake cycle in response to environmental time cues. These individuals' circadian clocks might have an unusually long cycle, and/or might not be sensitive enough to time cues. People with DSPD (Delayed sleep phase disorder), more common than Non-24, do entrain to nature's 24 hours, but are unable to sleep and awaken at socially preferred times, sleeping instead, for example, from 4 a.m. to noon. According to doctors Cataletto and Hertz at WebMD, "Altered or disrupted sensitivity to zeitgebers is probably the most common cause of circadian rhythm disorder."
Circadian rhythm abnormalities are also extremely common in ADHD, especially in the form of delayed sleep (sleep initiation insomnia). It has been genetically linked by findings of polymorphism in genes in common between those apparently involved in ADHD and those involved in the circadian rhythm and a high proportion of DSPD among those with ADHD, however no specific or further cause-effect relationship has been proven.
Treatment for circadian rhythm sleep disorders
Possible treatments for circadian rhythm sleep disorders include:
Behavior therapy or advice about sleep hygiene where the patient is told to avoid naps, caffeine, and other stimulants. They are also told to not be in bed for anything besides sleep and sex.
Bright light therapy is used to advance or delay sleep, depending on how the circadian rhythm is shifted. Patients are exposed to high-intensity light from a light box (up to 10,000 lux) or wearable light therapy device (up to 500 lux) for a duration of 30–60 minutes at a time, the time of day depending on whether an advance or a delay is required.
Blue blocking glasses therapy is used to block blue wavelength light from reaching the eye during evening hours so that melatonin production is increased.
Medications such as melatonin and modafinil (Provigil), or other short term sleep aids or wake-promoting agents can be beneficial; the former is a natural neurohormone responsible partly and in tiny amounts for the human body clock. Tasimelteon has been proven effective in Phase III trials.
^“Adult attention-deficit hyperactivity disorder is associated with alterations in circadian rhythms at the behavioural, endocrine and molecular levels” - Molecular Psychiatry , (22 November 2011) | doi:10.1038/mp.2011.149 (PubMed ID: 22105622 pre pub)
^Van der Heijden KB, Smits MG, Van Someren EJ, Gunning WB (2005). "Idiopathic chronic sleep onset insomnia in attention-deficit/hyperactivity disorder: a circadian rhythm sleep disorder". Chronobiology International22 (3): 559–70. doi:10.1081/CBI-200062410. PMID16076654.