As most people are required to keep a regular schedule for work, school, or other social obligations, the first symptoms of N24 usually noticed are periodic night-time insomnia and excessive daytime sleepiness. Due to the cyclical nature of the disorder, some affected persons will tend to feel normal for periods of days to weeks when their body’s rhythm is synchronized with the rhythm of society around them. As the individual’s body once again desynchronizes from the rhythms of the light-dark cycle (or day-night cycle) and the obligations the individual with N24 is trying to maintain, the insomnia and excessive daytime sleepiness will return.
The sleep cycle of persons with N24 usually ranges from just over 24 hours (e.g. 24.1 hours) to as many as 28-30 hours in extreme cases. Cases with cycles less than 24 hours are extremely rare.
When allowed to sleep on their own cycle, some individuals with N24 will find relief of their symptoms of insomnia and fatigue, at the cost of the ability to maintain a schedule required for social and occupational requirements. However some people with N24 will continue to experience fatigue, grogginess, malaise and disrupted sleep on any schedule, possibly because of continued desynchronization of their internal circadian rhythms.
If N24 is not detected and addressed, and the person attempts to stay on a 24-hour schedule, the symptoms of chronic sleep deprivation will accumulate, such as excessive daytime sleepiness, fatigue, depression, difficulty concentrating, and memory problems. N24 can be severely disabling as it causes extreme difficulty for the individual attempting to maintain social and career obligations. Isolation and loneliness can also be issues due to periodically being awake when others are asleep.
All life on earth has evolved in conditions of a 24-hour day-night (light-dark) cycle. Organisms have evolved mechanisms to time their cellular and metabolic processes to anticipate this daily rhythm. As a result, within nearly all cells of the human body there is a biological clock based on a cycle of DNA and protein synthesis. Clock gene activity has been found within white blood cells and cells of the heart, brain, liver and many other tissues.
The individual cellular clocks run on a cycle that is close to 24 hours. This is known as a circadian rhythm (“circa-” = about and “dian” = pertaining to a day). But because the clocks are not exact, the clocks of individual cells can drift apart from each other or from the earth’s day-night cycle. To keep these clocks in time there is a master clock located in the brain. In the same way that the conductor of an orchestra keeps the musicians playing in time with each other, this master clock keeps the body’s cellular clocks to the same time cycle.
The master clock is located in what is called the suprachiasmatic nucleus (SCN), located in a part of the brain called the hypothalamus which regulates many basic body functions. The SCN is composed of about 10,000 closely networked cells whose rhythms are coordinated so that the firing rate of the cells varies together in a near-24-hour rhythm. The firing of SCN cells is then transmitted directly and indirectly to many other regions of the brain which then pass on this clock signal to the rest of the body by neurochemical and hormonal means.
Two of the most important rhythms driven by the clock signal are the body temperature cycle and the production of the hormone melatonin. The SCN regulates body temperature via connections to other areas of the hypothalamus. Body temperature varies in a wave-like pattern, which reaches a maximum during the day and a minimum (or nadir) during the night.
The SCN also sends a nerve signal that follows a complex poly-synaptic pathway via the cervical spinal ganglia to regulate the activity of the pineal gland, which is responsible for the production of melatonin. Melatonin, sometimes called “the hormone of darkness,” is produced during the dark at night. It is secreted by the pineal into the cerebrospinal fluid and then travels in the bloodstream to reach the cells of the body. It acts upon specific melatonin receptors to directly regulate cell functions. It also reinforces the temperature cycle by causing a drop in body temperature at night. Among other effects, this drop in body temperature helps ready the brain and body for sleep.
While the SCN serves to coordinate the cell clocks throughout the body, there is still a need to coordinate the SCN clock to the earth’s 24-hour period. If left to itself, the SCN keeps a rhythm that is close to but not exactly 24 hours. In healthy humans the intrinsic period of the SCN clock averages about 24.2 hours. If there were no way to correct this cycle to equal 24 hours the clock in the SCN would drift by several minutes each day until it no longer kept correct time or stayed “entrained”.
The primary means to keep the SCN clock set properly is via light. Specialized cells in the retina of the eye, which are different from the cells used for vision, register the exposure to light and transmit this signal by a nerve path known as the retinohypothalamic tract to the SCN. When the eyes are exposed to light in the early morning hours they send a signal that advances the clock in the SCN to an earlier time. When light falls on the eyes late at night, a delay signal is sent to the SCN. A graph of the effect of light at different times of day and night is known as a phase-response curve and can be used to predict the effects of light on the biological clock. If the SCN clock runs longer than 24 hours, and becomes delayed relative to the day-night cycle, early morning light exposure will reset it. If the SCN clock runs shorter than 24 hours, late night light exposure will delay it a bit. By this means, the SCN clock is kept in time with the light and dark cycle of day and night. In healthy individuals routine exposure to morning light works to keep circadian rhythms entrained.
The retinal cells that register light for circadian functions use a pigment known as melanopsin as a light sensor. Because melanopsin is particularly sensitive to blue light, light of that color has a greater effect in circadian rhythms. Red, orange and yellow light have much less effect. Green light can also affect rhythms under certain circumstances.
Among the most important of the body rhythms controlled by the SCN is that of the sleep-wake cycle. This cycle is controlled by two processes known as the homeostatic process and the circadian process. During sleep the brain and body repair themselves and accumulate energy and metabolic resources for the activities of the day. During the day, while the person is awake, these resources are gradually consumed. The gradual loss of energy during the day produces a drive to sleep in order to restore that energy. This is known as the homeostatic sleep drive. If the homeostatic process were the only one involved, a person would wake up fully energized and then gradually wind down over the course of the day, like a battery losing power. This would mean an uneven level of alertness during the day, with dangerously low alertness in the afternoon and evening. To counterbalance this, the SCN also regulates alertness by what is known as the circadian process. As the day goes on, and energy winds down, the SCN compensates for this by sending a stronger alertness signal to the brain and body. This alertness signal reaches a peak in the two hours just before bedtime. This zone of maximum alertness is known as the “forbidden zone for sleep” since the alertness signal makes sleep nearly impossible during that zone. When the usual bedtime is reached, the SCN begins to turn down its alertness signal to allow the body to sleep. In order to prevent early awakening, before the night’s sleep is done, the circadian alertness signal declines further across the night.
This complex interplay between the circadian process and the homeostatic process allows the human organism to have a relatively level state of alertness during the day (with the occasional exception of a mid-afternoon nap period) and allows a 7-9 hour period of consolidated sleep at night.
When all is working well, light signals registered in the eyes keep the SCN on track with the 24-hour day-night cycle and the SCN in turn coordinates the clocks in the pineal gland and in cells throughout the body. All the clocks keep a 24-hour cycle in sync with each other like the members of a well-conducted orchestra. The circadian alertness signal then combines with the homeostatic process resulting in an individual who can sleep through the night and maintain alertness during the day.
But there are a number of things that can go wrong with this system and result in a circadian disorder such as N24.
1. Blindness. The most well-understood cause of N24 is what occurs in blind individuals. Persons who are completely blind (no perception of light) will not register the light signals which are needed to fine-tune the body clock to a 24-hour day. If the SCN clock starts to drift away from 24 hours, a blind person has no intrinsic way to bring it back in sync without medical treatment. Since the inherent rhythm of the SCN is not always precisely 24 hours, a blind person’s circadian timing system will slowly drift over time. They will cycle over time between periods of nighttime sleep and periods of daytime sleep. In the vast majority of cases the sleep rhythm gradually delays so the period is over 24 hours, but there are a few cases of gradual advances and a less-than-24-hour period. The length of the circadian period in blind persons with N24 is typically in the range of 23.8 to 25 hours.
2. Alterations in Light Sensitivity. In some sighted individuals there may be a subsensitivity or insensitivity to the effects of light on the circadian system. The vision-producing areas of the eye and brain may function well, but the separate cell pathway that transmits the circadian light signal may not. If they are totally insensitive to the circadian effects of light, their condition, from a circadian point of view, is not different from that of a blind person. If they are subsensitive to light, light may produce some effect on their rhythms but it may not be strong enough to correct circadian drift in their particular lighting environment.
Conversely, some patients with delayed sleep phase disorder, a condition related to N24, have been shown to be supersensitive to the effects of light. If they are exposed to normal room light in the evening it may produce an exaggerated delay in their circadian rhythms. If this delay becomes cumulative, the result is N24.
3. Environment. Environmental exposure to light may also play a role. Healthy individuals, when kept in isolation without time cues and allowed to turn their lights on and off when they choose, will often fall into a non-24-hour rhythm. The length of the rhythm is not only longer than the intrinsic 24.2-hour cycle of the SCN, but may be up to 25 hours or more in length. This is because self-selected light exposure late in the day has a delaying effect. However this cannot be the sole cause of N24 since light does not lead to N24 in all persons in a non-isolated environment. In contrast, persons with N24 cannot maintain a 24-hour schedule even in a non-isolated environment with normal time cues.
4. Hormonal Factors. In some cases the hormone melatonin may be involved in the development or perpetuation of N24. Some patients with N24 produce less melatonin than normal, which can be problematic since melatonin helps link sleep to the day-night cycle. On the other hand too much melatonin could also cause problems. It is known that some drugs that increase melatonin can lead to delayed sleep phase disorder, which is closely related to N24. Some individuals have an abnormality in their ability to metabolize melatonin, which can lead to higher-than-normal daytime levels that may result in circadian clock dysfunction.
5. Individual Differences in Circadian and Sleep Physiology. Other studies of the causes of circadian rhythm disorders have focused on the cellular clock itself. Although there are no direct studies of the cellular clock mechanism in N24 patients, studies in healthy subjects show a correlation between the period of the cellular clock and the phase of entrainment. Morning persons have a shorter clock period than evening persons. In addition there is a correlation between cellular clock length and overall body clock period length as determined in free-running forced desynchronization studies. N24 may be an extension of extreme “eveningness” in which the cellular rhythm may be too far from 24 hours for normal light exposure to correct it, a situation known as being “outside the range of entrainment”.
Another possible set of causes of N24 is related to the homeostatic and circadian regulation of sleep. On average patients with N24 have a slightly greater sleep requirement than normal. In some cases this can be extreme. While a healthy person may sleep 8 hours and be awake for 16 hours, if someone needs 12 hours of sleep and then is awake for a normal 16 hours, their day will last 28 hours total. The change in the sleep cycle will in turn change the timing of light exposure, perpetuating an N24 cycle. Similarly if someone is deficient in the homeostatic drive for sleep they may sleep a normal 8 hours but require 20 hours of awake time before sufficient homeostatic pressure accumulates to permit sleep, again resulting in a 28 hour day.
The timing of sleep in relation to internal circadian rhythms, also known as the phase angle between sleep and circadian rhythms, is abnormal in many cases of N24. Here phase angle is described in terms of the relationship between sleep timing and the circadian rhythm of body temperature. In healthy individuals the body temperature starts to drop shortly before sleep onset and reaches a minimum late in the sleep period — usually about 2 hours before waking. Persons with N24 tend to fall asleep very late relative to their temperature cycle and so the time between the temperature minimum and time of waking (sleep offset) may be 4 hours or more, even up to 8 hours in extreme cases. Since the body’s response to light-dark exposure is synched with the internal rhythms (such as core temperature) rather than the sleep-cycle per se, N24s with an abnormal relationship between sleep and circadian rhythms will sleep through the phase advance portion of their clock and not get the light they need on a daily basis to reset their clock. At the same time since they are awake late relative to their temperature cycle, they are exposed to light during the phase delay portion of the phase response curve. This tends to push their circadian rhythm in the direction of a much longer than normal day.
The circadian regulation of sleepiness is also important. Even healthy individuals have a “forbidden zone for sleep” that occurs an hour or two before normal bedtime and is associated with the maximum circadian alertness signal. In persons with N24 this forbidden zone occurs too late in the day and is too strong to permit sleep on a 24-hour cycle.
This pattern may be reinforced by certain effects of sleep and wake on alertness. When individuals wake after a prolonged period of sleep, they are often in a state of reduced alertness known as sleep inertia. In people with N24 this state of sluggishness and grogginess may be very powerful and persist for many hours. The longer they are awake the more alert they become. (This may be explained by an observation that brain cell circuits become more excitable with longer time awake.) When it comes time for them to sleep (if they are trying to stay on a 24-hour cycle) their alertness will have reached a high point and their heightened state of energy, even if brief, will not permit them to fall asleep at a normal time. In addition, patients with N24 may not want to try to fall asleep at
this time because they finally feel awake, alert and productive.
The abnormalities listed above that contribute to N24 may come about in different ways. Genetics is one factor. The basic timing of the biological clock is under genetic control and animals have been bred to exhibit non-24-hour cycles. In human N24 genetics is likely to be a factor, but since most N24s do not have parents with N24 (although some have parents with other circadian disorders) a simple genetic cause is not likely in most cases. It may be that multiple genes are involved.
6. Development. Development of the brain, and in particular the circadian and sleep centers, is another factor. In pervasive developmental disorders such as autism a relatively high frequency of occurrence of N24 and other circadian rhythm and sleep disorders has been noted. It is assumed that the circadian and sleep centers of the brain did not properly develop or are affected by other neurochemical or anatomical deficits. It may be that other N24s who do not have pervasive developmental disorders may have impaired development limited to the sleep and circadian brain centers.
7. Trauma. Physical damage to the brain, such as occurs from head injury has been noted to lead to N24 in previously healthy individuals. It is assumed that the head injury damages the sleep and circadian centers of the brain such as the hypothalamus or pineal gland. Similarly, brain tumors have been noted to lead to the development of N24. Circadian sleep disorders have been noted in survivors of tumors affecting the pons and the hypothalamus. Craniopharyngiomas are particularly likely to lead to sleep disorders. In some cases the damage is due to the tumor itself and in other cases to the effects of radiation treatment to the head. In one case an aneurysm near the SCN resulted in transient N24.
Under the heading of physical abnormalities, any factor that leads to total blindness, whether via genes, disease or injury, can lead to secondary N24.
8. Iatrogenic. N24 can also arise from attempts at treatment of the more common disorder, delayed sleep phase disorder (DSPD). One of the widely used treatments for DSPD is chronotherapy, in which the patient is instructed to gradually delay their bedtime and wake time up to three hours a day until they go around the clock to a more socially acceptable sleep-wake schedule. In essence this means temporarily adopting an N24 schedule. Unfortunately, in some patients, once an N24 schedule has been established it becomes nearly impossible to break. They have exchanged one circadian rhythm disorder, DSPD, for an even more disabling one, N24. There are several reasons why the N24 pattern is hard to break out of once established. One involves the timing of sleep relative to the temperature rhythm mentioned above. The other involves what is called the plasticity of the circadian system. That means that once an organism has been placed on a particular cycle, including a non-24-hour cycle, the circadian clock remembers that cycle and tries to continue it. The risk of N24 after chronotherapy has been known since the 1990s but many doctors continue to be unaware of the risk when recommending chronotherapy.
While the total number of people living with N24 is unknown, researchers assume that more blind people are affected than sighted people. Approximately half of all people who are totally blind have N24. People who lack any light perception (for example those whose eyes are enucleated) are more likely to be affected than those with some retinal function. The frequency of N24 among the sighted is unknown but the world-wide medical literature provides case studies of roughly 100 sighted individuals with N24. Fifty-seven of these cases appear in a single Japanese study. The president of the CSD-N support group (see under “organizations”) counts approximately 35 sighted individuals with N24 as members of the organization and the surrounding online community. As the condition is not widely known, there may be a significant number of undiagnosed cases.
In published cases of sighted patients, around 75% are male, although it is not known if this is representative of the ratio in the overall patient population. Studies in healthy adults show that on average men have longer circadian periods than women. Among support groups the numbers of male and female patients are roughly equal. The most frequent age of onset is late teens or early twenties, although N24 can manifest at a much younger or older age. The disorder appears to be life-long. Insufficient data exists to determine whether N24 is progressive. Anecdotal evidence offered by long-term sufferers indicates a worsening of symptoms with age, along with an increase in the day length, however this may be due to the interaction between N24 and age-induced sleep disruptions. Clinical research on changes in the manifestation of N24 throughout the life cycle is absent at present.
N24 was first described in the medical literature by Eliott, Mills, and Waterhouse in 1970.
Initial diagnosis is based on home sleep logs kept by the patient that show a non-24-hour sleep pattern. This is usually more easily distinguished if the patient’s sleep times are not constrained by social or occupational obligations.
Confirmation of diagnosis may be obtained by the use of an actigraph, a device worn on the wrist that registers movement which is used to track the timing of sleep. The actigraph should be worn for sufficient time for the sleep cycle to complete at least one pass around the clock, typically several weeks.
Documenting a non-24-hour pattern of secretion of hormones such as cortisol or melatonin may be a useful confirmation of the diagnosis, though this procedure is currently more commonly used for research purposes.
Clinical Testing and Work-Up
Sleep logs and actigraphy are the main means for initial work up and follow up. Polysomnography (an overnight sleep study) is not necessary for diagnosis of N24 but may be used to rule out related disorders. For polysomnography to be useful, it must be done at a time when the patient’s cycle allows him or her to sleep.
In 2014, The U.S. Food and Drug Administration (FDA) approved Hetlioz (tasimelteon), a melatonin receptor agonist, to treat N24. Hetlioz, manufactured by Vanda Pharmaceuticals, Inc., is the first FDA approved treatment for the disorder. The effectiveness of Hetlioz was evaluated in two clinical trials of totally blind individuals with N24.
The most widely recommended treatments for sighted patients involve exposure to specific regimens of light (phototherapy) and dark (scototherapy).
Phototherapy usually involves the use of a lightbox. The lightbox is used in the early morning, typically for a duration of 2 hours, in order to stabilize the sleep cycle. Light treatment is best started when the patient’s cycle already has them arising at the desired wake time. Light is registered by special cells in the retina of the eye which send a signal to the brain via the retinohypothalamic tract. This signal suppresses the output of melatonin and shifts the timing of sleep. A phase-response curve determines the best time for light exposure.
Dark therapy (scototherapy) is accomplished by avoiding light exposure late in the day. Even ordinary room light may have phase-delaying effect so patients should remain in dim light or use special dark goggles that reduce light exposure during the evening and night.
A combination of light and dark therapy is believed to be more effective than either alone. If entrainment to a 24-hour cycle is achieved with light and dark therapy, the patient must maintain the treatment regimen or entrainment will be lost.
The hormone melatonin may be used to stabilize the sleep-wake cycle. Melatonin is usually taken about 4-6 hours before the desired sleep time. While melatonin is often effective in blind patients with N24, it is rarely successful as the sole treatment in sighted patients.
Early case reports suggested that vitamin B12 could successfully treat some cases of N24; however, a double-blind placebo-controlled trial found it was not significantly better than placebo for treatment of N24 or DSPD.
Blue light plays a particular role in affecting circadian rhythms. Blue-enriched light has been used in treatment of the related condition, DSPD, and may be useful for N24, although there are no published cases or trials.
Conversely, avoidance of blue light using goggles which block out all blue (and sometimes green) light has become a widely used treatment among patients with N24 with anecdotal success, but as of yet there are no published studies of this approach. In addition to, or in place of goggles, patients may use special red or amber lights (which do not put out blue or green light) in the evening for illumination. They do not use standard room light and avoid sunlight by using shades or shutters in the evening.
There is considerable ongoing research on the basic biology and molecular genetics of circadian rhythms. Drugs which alter the timing of the biological clock are a promising avenue for future study but as of yet none are near being ready for clinical use. Research on the circadian and homeostatic control of sleep timing in healthy subjects and patients with N24 and related disorders may also offer clues to future treatments.
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Tollfree: (800) 411-1222
TTY: (866) 411-1010
For information about clinical trials sponsored by private sources, contact:
For information about clinical trials conducted in Europe, contact:
Kryger MH et al., eds. Principles and Practice of Sleep Medicine. 5th ed. St. Louis, MO: Elsevier, Inc.; 2011:470-482.
American Academy of Sleep Medicine. International Classification of Sleep Disorders, 2nd ed.: Diagnostic and Coding Manual. Westchester, IL: American Academy of Sleep Medicine, 2005:126-136.
Meier-Ewert K, Okawa M. Sleep-Wake Disorders. New York, NY: Plenum Press; 1997:53-66.
Zhu L, Zee PC. Circadian rhythm sleep disorders. Neurol Clin. 2012 Nov;30(4):1167-91.
Huber R, Mäki H, Rosanova M, et al. Human Cortical Excitability Increases with Time Awake [published online ahead of print February 7 2012]. Cereb Cortex. 2012.
http://cercor.oxfordjournals.org/content/early/2012/02/07/cercor.bhs014.full.pdf+html. Accessed December 31, 2012.
Duffy JF, Cain SW, Chang AM, et al. Sex difference in the near-24-hour intrinsic period of the human circadian timing system. Proc Natl Acad Sci U S A. 2011;13;108 Suppl 3:15602-8.
Uchimaya M, Lockley SW. Non-24-hour sleep-wake syndrome in sighted and blind patients. Sleep Med Clin 2009;4:195-211.
Pagani L, Semenova EA, Moriggi E, et al. The physiological period length of the human circadian clock in vivo is directly proportional to period in human fibroblasts. PLoS One 2010;5(10):e13376.
Morgenthaler TI, Lee-Chiong T, Alessi C, et al. Standards of Practice Committee of the American Academy of Sleep Medicine. Practice parameters for the clinical evaluation and treatment of circadian rhythm sleep disorders. An American Academy of Sleep Medicine report. Sleep 2007;30(11):1445-59.
Okawa M, Uchiyama M. Circadian rhythm sleep disorders: characteristics and entrainment pathology in delayed sleep phase and non-24 sleep-wake syndrome. Sleep Medicine Reviews 2007;11:485-496.
Sack RL, Auckley D, Auger RR, et al. Circadian rhythm sleep disorders: part II, advanced sleep phase disorder, delayed sleep phase disorder, free-running disorder, and irregular sleep-wake rhythm. An American Academy of Sleep Medicine review. Sleep 2007;30(11):1484-501.
Dagan Y, Ayalon L. Case study: psychiatric misdiagnosis of non-24-hours sleep-wake schedule disorder resolved by melatonin. J Am Acad Child Adolesc Psychiatry 2005;44(12):1271-5.
Hayakawa T, Uchiyama M, Kamei Y, et al. Clinical analyses of sighted patients with non-24-h sleep wake syndrome: a study of 57 consecutively diagnosed cases. Sleep 2005;28(8):949.
Boivin DB, Caliyurt O, James FO, et al. Association between delayed sleep phase and hypernyctohemeral syndromes: a case study. Sleep 2004;27(3):417-21.
Boivin DB, James FO, Santo JB, et al. Non-24-hour sleep-wake syndrome following a car accident. Neurology 2003;60:1841-3.
Dagan Y. Circadian Rhythm Sleep Disorders (CRSD) in psychiatry–a review. Isr J Psychiatry Relat Sci 2002;39(1):19-27.
Uchiyama M, Shibui K, Hayakawa T, et al. Larger phase angle between sleep propensity and melatonin rhythms in sighted humans with non-24-hour sleep-wake syndrome. Sleep 2002;25:83-8.
Dagan Y, Abadi J. Sleep-wake schedule disorder disability: a lifelong untreatable pathology of the circadian time structure. Chronobiol Int 2001 Nov;18(6):1019-27.
Shibui K, Uchiyama M, Iwama H, et al. Periodic fatigue symptoms due to desynchronization in a patient with non-24-h sleep-wake syndrome. Psychiatry Clin Neurosci 1998;52:477-81.
Oren DA, Giesen HA, Wehr TA. Restoration of detectable melatonin after entrainment to a 24-hour schedule in a ‘free-running’ man. Psychoneuroendocrinology 1997;22(1):39-52.
McArthur AJ, Lewy AJ, Sack RL. Non-24-hour sleep-wake syndrome in a sighted man: circadian rhythm studies and efficacy of melatonin treatment. Sleep 1996;19:544-53.
Uchiyama M, Okawa M, Ozaki S, et al. Delayed phase jumps of sleep onset in a patient with non-24-hour sleep-wake syndrome. Sleep 1996;19:637-40.
Yamadera H, Takahashi K, Okawa M. A multicenter study of sleep-wake rhythm disorders: clinical features of sleep-wake rhythm disorders. Psychiatry Clin Neurosci 1996;50:195-201.
Yamadera H, Takahashi K, Okawa M. A multicenter study of sleep-wake rhythm disorders: therapeutic effects of vitamin B12, bright light therapy, chronotherapy and hypnotics. Psychiatry Clin Neurosci. 1996;50(4):203-9.
Oren AD, Wehr TA. Hypernyctohemeral syndrome after chronotherapy for delayed sleep phase syndrome. N Engl J Med. 1992;327(24):1762.
Hoban TM, Sack RL, Lewy AJ, et al. Entrainment of a free-running human with bright light? Chronobiol Int 1989;6:347-53.
Eastman CI, Anagnopoulos CA, Cartwright RD. Can bright light entrain a free-runner? Sleep Res 1988;17:372.
Sugita Y, Ishikawa H, Mikami A, et al. Successful treatment for a patient with hypernychthemeral syndrome. Sleep Res 1987;16:642.
Wollman M, Lavie P. Hypernychthemeral sleep-wake cycle: some hidden regularities. Sleep 1986;9: 324-34.
Kamgar-Parsi B, Wehr TA, Gillin JC. Successful treatment of human non-24-hour sleep-wake syndrome. Sleep 1983;6:257-64.
Weber AL, Cary MS, Connor N, et al. Human non-24-hour sleep-wake cycles in an everyday environment. Sleep 1980;2:347-54.
Kokkoris CP, Weitzman ED, Pollak CP, et al. Longterm ambulatory temperature monitoring in a subject with a hypernychthemeral sleep-wake cycle disturbance. Sleep 1978;1:177-90.
Eliott AL, Mills JN, Waterhouse JM. A man with too long a day. J Physiol 1970;212:30-1.
DSPS – A Sleep Disorder. Charting the Course of N24. http://delayed2sleep.wordpress.com/2010/10/27/60-charting-the-course-of-n24/
Updated October 27, 2010. Accessed:March 13, 2013.