Jürgen Walther Ludwig Aschoff (January 25, 1913 – October 12, 1998) was a German physician, biologist and behavioral physiologist. Together with Erwin Bünning and Colin Pittendrigh, he is considered to be a co-founder of the field of chronobiology.
Aschoff's work in the field of chronobiology introduced ideas of light interactions in the circadian rhythms of nocturnal and diurnal species as summarized by Aschoff's Rules.
Life Aschoff was born in Freiburg Im Breisgau, the fifth child of pathologist Ludwig Aschoff (known for discovering the Aschoff-Tawara or atrioventricular node) and his wife Clara. He grew up in the liberal but morally strict world of Prussian academia. After the Abitur at a humanistic high school, he – according to his own statement "lacking a specific interest" – studied medicine at the University of Bonn, where he joined the Burschenschaft (fraternity) Alemannia Bonn. Aschoff's scientific career began in 1938, when he moved to the University of Göttingen to study thermoregulation physiology with Hermann Rein. In 1944, he received the venia legendi. He then became a professor at the University of Göttingen in 1949.
In 1952, his mentor, Hermann Rein, was appointed director of the Max Planck Institute for Medical Research in Heidelberg. Rein brought Aschoff to the Institute as a collaborator to study circadian rhythms in humans, birds, and mice. Aschoff then moved to the Max Planck Institute for Behavioral Physiology in Andechs to work with Gustav Kramer, who showed time-compensated sun-compass navigation in birds, and Erich von Holst, who studied physiological oscillators. From 1967 to 1979, he was a director at the Max Planck Institute for Behavioral Physiology and a professor in Munich. Aschoff was a scientific member and a member of the Kollegiums of the Max Planck Institute for Behavior Physiology, as well as senator of the Max Planck Society from 1972 to 1976.
Aschoff was known as an excellent lecturer with a booming voice, and he took a special interest in creating a scientific community and encouraging young scientists. After his retirement in 1983 and return to Freiburg, Aschoff continued his scientific work in the form of further publications. Jürgen Aschoff died in 1998 aged 85 after a short illness, only ten months after his beloved wife, Hilde, died.
Work Aschoff provided a strong foundation for the field of chronobiology through his research on circadian rhythms and entrainment in many different organisms such as rats, mice, birds, macaques, monkeys, and humans. His early research focused on understanding the properties of circadian rhythms and how these rhythms can change in response to stimuli. His later work was more applicable to pathologies, such as psychiatric disorders and dangers of shift work schedules, which can result from manipulating specific Zeitgebers.
Early work Aschoff began his research on the physiology of thermoregulation by self-experimentation. He discovered that there was a 24-hour rhythm of variation in body temperature. After these experiments, he began investigating the basic mechanisms of circadian rhythm. In the 1950s, he met and began to collaborate with Erwin Bünning and Colin Pittendrigh. Aschoff began further experimentation studying the circadian rhythms of birds and mice under constant conditions. His results led to the conclusion that the circadian oscillations of biological processes were innate and did not require prior exposure to a 24-hour day to be expressed.
Aschoff also applied these methods to experiments with human circadian rhythms by building an underground "bunker" to isolate human subjects from any external environmental cues. Subjects placed in this bunker were allowed to turn lights on or off according to their own internal rhythms. After over twenty years of tracking sleep-wake cycles, body temperature, urine output, and other physiological and behavioral outputs, Aschoff and his collaborator Rütger Wever concluded that humans have endogenous circadian oscillators. This discovery has become the foundation for our understanding of many medical problems such as aging, sleep disorders, and jet lag.
Colin Pittendrigh is commonly thought to be the first researcher to study chronobiology in modern times. He studied fruit flies in the 1950s, and his mentee, Jürgen Aschoff, studied humans in the 1960s. In 1960, Aschoff coined the term Zeitgeber (from German for "time giver" or "synchronizer") to refer to external, environmental cues that synchronize an endogenous oscillator to the environmental cycle. To investigate the properties of natural endogenous oscillators, Aschoff exposed organisms to constant conditions without Zeitgeber cues (either constant light or constant darkness). The observations from this paper were formulated into the fundamental rules of biological clocks.
Aschoff's Rule
History Aschoff's experiments executed in 1960 showed that under constant light conditions, the activity phase shortens in nocturnal organisms and lengthens in diurnal organisms. These trends were termed alpha compression and alpha expansion, respectively. In tribute to his mentor, Pittendrigh called this observation "Aschoff's Rule" in a different 1960 publication, and the designation remains today.
Aschoff's rule is related to the model of parametric entrainment, which assumes continuous phase changes. This means that activity patterns adjust to environmental light patterns. Parametric and non-parametric concepts refer to the nature of the action of light, whether continuous or tonic in the case of parametric entrainment, or discrete or phasic in the case of nonparametric entrainment. Suggestive of phasic effects are the large phase shifts elicited often by very brief light pulses, while the changes in period as a result of different constant light intensities are proof of the existence of tonic effects. Aschoff and Pittendrigh approached the field with different models of how oscillators entrain, which resulted in different predictive models.Daan, Serge. "Colin Pittendrigh, Jürgen Aschoff, and the Natural Entrainment of Circadian Systems." The Colin S. Pittendrigh Lecture, May 9, 1998, accessed April 20, 2011. Aschoff's parametric model states that entrainment occurs through gradual changes in the clock that adapt to a new light-dark cycle. Although this is no longer recognized as the only objectively correct model in the field, Serge Daan suggested in 1998 that Aschoff made qualitative contributions that provide valuable alternatives to inconsistencies in the current field.
Terms Relevant terms to understanding Aschoff's Rule are listed below:
Actogram: A graph of the phases of activity and rest over the course of a day. Alpha: The duration of wake time. Alpha Compression: The observation that under constant light conditions, the length of activity of nocturnal organisms shortens. Alpha Expansion: The observation that under constant darkness conditions, the length of activity of nocturnal organisms lengthens. Circadian: Any rhythm that has a period of approximately 24h. Circadian Clock: The endogenous, molecular mechanism which regulates circadian rhythms. Circadian Time (CT): Time defined by an organism's intrinsic clock without accounting for Zeitgebers. Diurnal: An organism that displays activity during the day (in light conditions) and during subjective day (in constant conditions) Endogenous: Growing or working from within an organism; intrinsic. Entrainment: The process an organism undergoes when synchronizing endogenous rhythms to a Zeitgeber. Free-Running Period (FRP): The length of time it takes for an organism's endogenous rhythm to repeat without Zeitgebers in constant conditions (LL or DD). Masking: The apparent coupling of an observable rhythm in an organism to a Zeitgeber, but this change is not caused by an alteration of the endogenous clock that schedules the observable rhythm, but by a confounding variable. Nocturnal: An organism that is active during the night (in light conditions) and during subjective night (in constant conditions). Onset of Activity: The time when the passive phase ends, and the active phase begins. Phase-Shift: A change in free-running cycle, caused by an external stimulus. Phase Response Curve (PRC): A graph representing the ways that external stimuli can affect phase. Photoperiod: (a.k.a. Day Length): The duration of light in an external light-dark cycle. Splitting: A phenomenon that occurs in high-intensity, constant light conditions, where two separate bouts of activity can be observed in one period. Tau 'τ': The length of an organism's free-running period. Tau 'Τ': The total period of an external period, environmental period. Zeitgeber: Any external time cue that is effective in entraining an organism. Zeitgeber Time (ZT): Time defined by external stimuli.
Rules Aschoff's 4 Rules are generalities that most diurnal and nocturnal organisms follow. The main basis for Aschoff's rule was the differential responses in free-running periods to DD and LL, later expanded to Tau=f(LL-intensity). These rules provide an explanation of how the free running period (Tau 'τ') of an organism deviates slightly from a 24-hour period.
Rule 1: The free running period of a nocturnal organism is typically less than 24 hours.𝜏DD < 24.
Rule 2: The free running period of a diurnal organism is typically more than 24 hours.𝜏LL > 24
Rule 3: Increasing the intensity of a light in LL will typically increase the length of a free running period in nocturnal organisms.
Rule 4: Increasing the intensity of a light in LL will typically decrease the length of a free running period in diurnal organisms.
To help with understanding, a nocturnal species such as house mice that are kept in a constantly dark environment would exhibit a free-running period shorter than 24 hours (Aschoff's first rule). In contrast, a diurnal species such as a starling that is kept in a constantly dark environment would exhibit a free-running period longer than 24 hours (Aschoff's second rule).
There are a few exceptions to these rules. For example, certain species of ground beetles and squirrels violate Aschoff's first rule by not producing the predicted changes in their free-running rhythms in response to constant light (LL). Some arthropods also appear to violate Aschoff's third rule however it is not certain because prior lighting history can cause long-lasting after-effects on the circadian period and alter the observed compliance with Aschoff's Rules.
Later work Much of Aschoff's later work involved tests on human subjects. He found that the absence of a light-dark cycle does not prevent humans from entrainment. Rather, knowing the time of day from social cues, such as regular meal times, is sufficient for entrainment. Aschoff also found that different circadian outputs such as body temperature and locomotor activity can be either internally synchronized or desynchronized depending on the strength of the Zeitgeber. In constant darkness, rectal temperature and sleep onset and duration became desynchronized in some subjects, and the rectal temperature at the time of sleep onset was correlated to the duration of the bout of sleep. He hypothesized that internal desynchronization, the phase differences resulting from period differences between two circadian output processes, could be related to many psychiatric disorders.