Free-running sleep is a rare sleep pattern whereby the sleep schedule of a person shifts later every day. It occurs as the sleep disorder non-24-hour sleep–wake disorder or artificially as part of experiments used in the study of circadian and other rhythms in biology. Study subjects are shielded from all time cues, often by a constant light protocol, by a constant dark protocol or by the use of light/dark conditions to which the organism cannot entrain such as the ultrashort protocol of one hour dark and two hours light. Also, limited amounts of food may be made available at short intervals so as to avoid entrainment to mealtimes. Subjects are thus forced to live by their internal circadian "clocks".
A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism and responds to the environment. Circadian rhythms are regulated by a circadian clock whose primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximise the fitness of an individual. Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.
Non-24-hour sleep–wake disorder is one of several chronic circadian rhythm sleep disorders (CRSDs). It is defined as a "chronic steady pattern comprising [...] daily delays in sleep onset and wake times in an individual living in a society". Symptoms result when the non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the light–dark cycle in nature. Although this sleep disorder is more common in blind people, affecting up to 70% of the totally blind, it can also affect sighted people. Non-24 may also be comorbid with bipolar disorder, depression, and traumatic brain injury. The American Academy of Sleep Medicine (AASM) has provided CRSD guidelines since 2007 with the latest update released in 2015.
Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of an additional photoreceptor was first suspected in 1927 when mice lacking rods and cones still responded to changing light levels through pupil constriction; this suggested that rods and cones are not the only light-sensitive tissue. However, it was unclear whether this light sensitivity arose from an additional retinal photoreceptor or elsewhere in the body. Recent research has shown that these retinal ganglion cells, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light-sensitive protein. Therefore, they constitute a third class of photoreceptors, in addition to rod and cone cells.
Circadian rhythm sleep disorders (CRSD), also known as circadian rhythm sleep-wake disorders (CRSWD), are a family of sleep disorders which affect the timing of sleep. CRSDs arise from a persistent pattern of sleep/wake disturbances that can be caused either by dysfunction in one's biological clock system, or by misalignment between one's endogenous oscillator and externally imposed cues. As a result of this mismatch, those affected by circadian rhythm sleep disorders have a tendency to fall asleep at unconventional time points in the day. These occurrences often lead to recurring instances of disturbed rest, where individuals affected by the disorder are unable to go to sleep and awaken at "normal" times for work, school, and other social obligations. Delayed sleep phase disorder, advanced sleep phase disorder, non-24-hour sleep–wake disorder and irregular sleep–wake rhythm disorder represents the four main types of CRSD.
Ignacio Provencio is an American neuroscientist and the discoverer of melanopsin, an opsin found in specialized photosensitive ganglion cells of the mammalian retina. Provencio served as the program committee chair of the Society for Research on Biological Rhythms from 2008 to 2010.
Light effects on circadian rhythm are the response of circadian rhythms to light.
Richard Ernest Kronauer was the Gordon McKay Professor of Mechanical Engineering, emeritus, at Harvard University. Though experienced with research in both fluid mechanics and applied mathematics.
Charles Andrew Czeisler is a Hungarian-American physician and sleep and circadian researcher. He is a leading researcher and author in the fields of the effects of light on human physiology, circadian rhythms and sleep medicine.
A chronobiotic is an agent that can cause phase adjustment of the circadian rhythm. That is, it is a substance capable of therapeutically entraining or re-entraining long-term desynchronized or short-term dissociated circadian rhythms in mammals, or prophylactically preventing their disruption following an environmental insult such as is caused by rapid travel across several time zones. The most widely recognized chronobiotic is the hormone melatonin, secreted at night in both diurnal and nocturnal species.
Rütger Wever was a German scientist, known for his significant contributions to the field of Chronobiology, including some of the first experiments on humans in time isolated environments.
Studies, which include laboratory investigations and field evaluations of population groups that are analogous to astronauts, provide compelling evidence that working long shifts for extended periods of time contributes to sleep deprivation and can cause performance decrements, health problems, and other detrimental consequences, including accidents, that can affect both the worker and others.
Derk-Jan Dijk is a researcher of sleep and circadian rhythms. As at 2024 he is a Distinguished Professor at the University of Surrey and the director of its Sleep Research Centre.
A constant routine protocol is a common method used in human circadian rhythm research to study internally generated, or endogenous, circadian rhythms without the effect of external, or exogenous, influences. In the method, subjects are kept in constant conditions for at least 24 hours. These include constant light and temperature, as well as constant semi-recumbent posture. In addition, subjects' food intake is evenly distributed throughout the protocol, and subjects are typically not allowed to sleep for the duration. While in these conditions, subjects are often assessed for a number of variables of interest. Two of the most common and best understood of these variables are core body temperature and melatonin.
Johanna H. Meijer is a Dutch scientist who has contributed significantly to the field of chronobiology. Meijer has made notable contributions to the understanding of the neural and molecular mechanisms of circadian pacemakers. She is known for her extensive studies of photic and non-photic effects on the mammalian circadian clocks. Notably, Meijer is the 2016 recipient of the Aschoff and Honma Prize, one of the most prestigious international prizes in the circadian research field. In addition to still unraveling neuronal mechanisms of circadian clocks and their applications to health, Meijer's lab now studies the effects of modern lifestyles on our circadian rhythm and bodily functions.
Dr. Debra J. Skene is a chronobiologist with specific interest in the mammalian circadian rhythm and the consequences of disturbing the circadian system. She is also interested in finding their potential treatments for people who suffer from circadian misalignment. Skene and her team of researchers tackle these questions using animal models, clinical trials, and most recently, liquid chromatography-mass spectrometry. Most notably, Skene is credited for her evidence of a novel photopigment in humans, later discovered to be melanopsin. She was also involved in discovering links between human PER3 genotype and an extremely shifted sleep schedules categorized as extreme diurnal preference. Skene received her Bachelor of Pharmacy, Master of Science, and Ph.D. in South Africa.
Ken-Ichi Honma is a Japanese chronobiologist who researches the biological mechanisms underlying circadian rhythms. After graduating from Hokkaido University School of Medicine, he practiced clinical psychiatry before beginning his research. His recent research efforts are centered around photic and non-photic entrainment, the structure of circadian clocks, and the ontogeny of circadian clocks. He often collaborates with his wife, Sato Honma, on work involving the mammalian suprachiasmatic nucleus (SCN).
Charmane Eastman is an American academic research scientist whose career has focused on studying circadian rhythms and their relationships to sleep, jet lag, and shift work. She has also studied winter depression, more properly known as seasonal affective disorder (SAD). Of special focus are the effects of bright light and melatonin on circadian rhythms.
Anne-Marie Chang is a researcher in the fields of sleep and circadian rhythms and is currently an Assistant Professor of Bio-behavioral Health at Pennsylvania State University. She earned her Ph.D. in neuroscience from Northwestern University in 2003.
