Herbie Murray
The sleep/wake cycle, also known as the circadian rhythm, is an important aspect of human biology that determines when we feel sleepy and when we are alert and awake. This internal body clock is influenced by various factors, including light and dark conditions, mealtimes, and our exposure to external time cues. When individuals are shielded from these external cues, their sleep patterns may shift, resulting in a rare condition called Non-24-hour sleep-wake disorder or free-running disorder (FRD). This disorder is characterised by a progressive delay in sleep timing, resulting in a sleep schedule that shifts later each day. The study of free-running sleep provides valuable insights into the complex nature of human sleep/wake cycles and how they can be influenced and treated.
| Characteristics | Values |
|---|---|
| Definition | A rare sleep pattern where the sleep schedule shifts later every day |
| Other Names | Non-24-hour sleep-wake disorder, Circadian Rhythm Sleep Disorder, Free-Running Type (CRSD, FRT), Free-Running Disorder (FRD), Non-24 |
| Cause | Lack of exposure to natural light and other social zeitgebers |
| Symptoms | Insomnia, excessive sleepiness, impairment in daytime function |
| Treatment | Melatonin, light therapy, increased sleep structure |
| Occurrence | Rare in sighted people, common in blind people |
| Age | Typically occurs in people younger than 30 years |
| Environment | Low light levels, atypical schedules |
| Psychiatric Disorders | Social phobia, OCD, MDD, anxiety disorders, depression, panic disorder, agoraphobia |
| Sleep Cycle Length | 28 hours |
| Sleep Cycle Stages | 4 to 6 sleep cycles per night, each lasting 90 minutes on average |
| Sleep Cycle Stages Breakdown | NREM sleep (N1, N2, N3), REM sleep |
| Chemicals Involved | Neurotransmitters (norepinephrine, histamine, serotonin, acetylcholine), melatonin, adenosine, dopamine |
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What You'll Learn
Circadian rhythm sleep disorder
CRSDs can be categorized into two groups based on their underlying mechanisms. The first category is composed of disorders where the endogenous oscillator has been altered, known as intrinsic type disorders. An example of this is the non-24-hour sleep-wake rhythm disorder (N24SWD), where a person's circadian rhythm extends slightly beyond the regular 24-hour schedule. This leads to bedtimes and wake times that shift later each day. The second category consists of disorders in which the external environment and the endogenous circadian clock are misaligned, called extrinsic type CRSDs. Blind people who cannot perceive light are most likely to develop this type of disorder, as their circadian rhythms cannot be influenced by light exposure. Less commonly, the disorder develops in people who are sighted but have unusually long circadian rhythms or atypical schedules with low light levels.
CRSDs can cause a persistent pattern of sleep/wake disturbances, resulting in disrupted rest and wakefulness. Those affected by CRSDs can fall asleep at unconventional times during the day or experience excessive daytime sleepiness if they resist. These occurrences often lead to recurring instances of disrupted sleep and wakefulness, where individuals affected by the disorder are unable to sleep and awaken at "normal" times for work, school, and other social obligations. CRSDs have also been associated with excessive daytime sleepiness and nighttime insomnia in patients diagnosed with Alzheimer's disease. Psychiatric disorders, and notably anxiety disorders, are also associated with CRSDs.
CRSDs can be diagnosed by a doctor who will ask about your sleep habits and may suggest a sleep study and other diagnostic tests. Treatment plans will depend on the type and cause of the CRSD. Melatonin, light therapy, and increased sleep structure have been shown to help in entraining and stabilizing the circadian rhythm.
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Non-24-hour sleep-wake disorder
The symptoms of Non-24 include a sleep pattern that varies from day to day, with different wake-up times and bedtimes. Individuals with this disorder may experience periods of sleeping during the day (symptomatic periods) alternating with periods of sleeping during normal nighttime hours (asymptomatic periods). They may also experience inappropriate fluctuations in appetite, mood, and alertness as their circadian rhythm becomes desynchronized from regular daylight hours.
The diagnosis of Non-24-hour sleep-wake disorder typically involves the use of sleep logs or diaries, actigraphy (wrist-worn devices that track movement and sleep patterns), and blood, urine, or saliva tests over several weeks to confirm the presence of a non-24-hour sleep pattern. Treatment options include phototherapy, melatonin therapy, medication, and chronotherapy, which involves gradually adjusting sleep and wake times. HETLIOZ® (tasimelteon) is the first and only FDA-approved treatment for Non-24 in totally blind individuals.
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Sleep-wake homeostasis
The pressure to sleep builds up in our bodies as our time awake increases, and the longer we stay awake, the stronger this pressure gets. It decreases during sleep, reaching a low after a full night of good-quality sleep. The homeostatic process begins to build again after we awaken. Our sleep drive is influenced by our immune system, with more immune mediators being produced when our body is fighting an infection, leading to increased sleepiness. Cognitively stimulating or physically demanding experiences can also increase sleep pressure, resulting in longer and deeper sleep.
Disruptions to our sleep or circadian rhythms can lead to peripheral disturbances that impact sleep homeostasis. For example, exposure to artificial light outside of daytime hours can disrupt our circadian rhythm and, in turn, our sleep drive. In rare cases, individuals may experience a sleep disorder called non-24-hour sleep-wake disorder, or free-running disorder (FRD), where their sleep schedule shifts later every day. This can occur due to a lack of exposure to natural light and other social cues, as well as underlying psychiatric disorders.
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Sleep stages
Sleep is divided into four stages, three of which are non-rapid eye movement (NREM) stages, and one of which is a rapid eye movement (REM) stage. Together, these four stages make up one sleep cycle, which typically occurs four to six times per night. The first sleep cycle is usually the shortest, ranging from 70 to 100 minutes, while later cycles are longer, lasting 90 to 120 minutes.
The first stage of the sleep cycle is NREM 1, which is the lightest stage of sleep. During this stage, the body and brain activities start to slow, but the body has not yet fully relaxed. It is easy to wake someone up during this stage, but if they are not disturbed, they will quickly move into the second stage. NREM 1 normally lasts just one to seven minutes, making up about 5% of total sleep time.
The second stage of the sleep cycle is NREM 2, which is still light sleep but deeper than the first stage. During this stage, brain waves slow down and have noticeable pauses between short, powerful bursts of electrical activity. Experts believe that these bursts are the brain organizing memories and information from the day. NREM 2 accounts for about 45% of total sleep time and is the stage that people spend the most time in.
The third stage of the sleep cycle is NREM 3, which is deep sleep. During this stage, brain waves are slow but strong, and the body takes advantage of this very deep sleep stage to repair injuries and reinforce the immune system. NREM 3 makes up about 25% of total sleep time in adults, but babies and children need more of this stage. Without enough NREM 3 sleep, one will feel tired and drained, even if they slept for a long time.
The fourth and final stage of the sleep cycle is REM sleep, during which the body moves into a state of dreaming. After the first REM cycle, the body starts a new sleep cycle and goes back into NREM 1 or NREM 2.
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Entrainment
The phasic mode is when there is limited interaction with the environment to "reset" the clock every day by an amount equal to the "error", which is the difference between the environmental cycle and the organism's circadian rhythm. The continuous mode is when the circadian rhythm is continuously adjusted by the environment, usually by constant light. Two properties, the free-running period of an organism, and the phase response curve, are the main pieces of information needed to investigate individual entrainment.
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Frequently asked questions
A free-running sleep-wake cycle is a rare sleep pattern where the sleep schedule shifts later every day. It is also known as non-24-hour sleep-wake disorder or free-running disorder (FRD).
The free-running sleep-wake cycle is a result of the intrinsic circadian rhythm no longer aligning with the 24-hour schedule. This can be due to a lack of exposure to natural light or other social cues that signal the time of day.
Symptoms of a free-running sleep-wake cycle can include insomnia, excessive sleepiness, and/or impairment in daytime function. It can also be associated with psychiatric disorders such as depression, anxiety, and agoraphobia.
A free-running sleep-wake cycle can be diagnosed through meticulous documentation of sleep patterns over an extended period. This can include recording estimated time in bed (TIB) and total sleep time (TST).
Treatment for a free-running sleep-wake cycle may include melatonin, light therapy, and increased sleep structure. It is important to get regular and adequate amounts of sleep to help balance out the sleep-wake cycle.
