Jürgen Walther Ludwig Aschoff (January 25, 1913 – October 12, 1998[1]) was a German physician, biologist and behavioral physiologist.

Aschoff's scientific career began in 1938, when he moved to the University of Göttingen to study thermoregulation physiology with Hermann Rein.

[2] In 1952, his mentor, Hermann Rein, was appointed director of the Max Planck Institute for Medical Research in Heidelberg.

[1] Rein brought Aschoff to the Institute as a collaborator to study circadian rhythms in humans, birds, and mice.

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.

Jürgen Aschoff died in 1998 aged 85 after a short illness, only ten months after his beloved wife, Hilde, passed away.

[2] 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 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.

[6] In tribute to his mentor, Pittendrigh called this observation "Aschoff's Rule" in a different 1960 publication, and the designation remains today.

[10] Aschoff's parametric model states that entrainment occurs through gradual changes in the clock that adapt to a new light-dark cycle.

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).

[17] Rule 4: Increasing the intensity of a light in LL will typically decrease the length of a free running period in diurnal organisms.

[17] 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).

[15] 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).

[20] 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.

Aschoff concluded that the oscillator or circadian clock “integrates” over the intensity of light to which it has been exposed, and then responds with a change in the period of activity, as seen in greenfinches, chaffinches, hamsters, and siskins.

[25] Jurgen Aschoff and Rütger Wever created an experiment in which participants lived in a bunker for multiple weeks in a row, with limited to no access to the outside world, to measure effects on bodily functioning, sleep-wake activity, and time perception.

[26] Free running periods in humans are thought to range from 23.56hrs-24.7hrs,[27] and it is suspected that the larger value found in this experiment is due to light exposure.

Aschoff's work inspired research related to blue light intensity, molecular basis of circadian rhythms, and modeling the SCN.

At the Chinese Academy of Sciences in Beijing, China in the Wang[30] lab, research was conducted to determine the role of blue light on CRY2 and PRR9 interactions in plants to create a molecular basis for Aschoff's rule.

‌In the Foster[31] lab at Imperial College in London, United Kingdom, research was conducted to determine the molecular basis for Aschoff's rule.

At the University of Aveiro, Portugal in the Yoon[32] lab researchers determined that the core-shell organization of the suprachiasmatic nuclei (Kuramoto model) contributes to anticipation and dissociation with regard to activity.

This allows the system to demonstrate anticipation of external events by peaking at two separate times and queueing peripheral clocks of the incoming signal.