The study may also explain why we have different levels of noise sensitivity during different times of the day.

Important body functions, such as sleep, the immune system, and hormone levels are controlled by a biological circadian clock.

 Now researchers at Karolinska Institutet in Sweden have discovered that there is also a biological clock in the hearing organ, the cochlea, controlled by genes known to regulate circadian rhythms.

One of these genes was found to cycle in the cochlea of mice over several days in a pattern that followed the hours of the day, researchers said.

By measuring the activity of the auditory nerve, they found that mice exposed to moderate noise levels during the night suffered from permanent hearing damages while mice exposed to similar noise levels during the day did not.

The ability to heal after hearing damage was therefore linked to the time of day during which the noise damage occurred, and here the ear's circadian clock played an important role.

It is known that the production of the growth hormone, BDNF, Brain-derived neurotrophic factor, known to protect auditory nerve cells, fluctuates throughout the day.

When mice were exposed to noise during daytime, their concentration of BDNF in the ear increased, which protected them from permanent hearing damage. This protective response was absent at night time.

 However, researchers succeeded in tricking the mice's earclocks in an experiment where they exposed mice to noise at night while stimulating BDNF at the same time.

Mice were then protected from permanent hearing loss as their auditory nerve cells successfully recovered from noise injury.

"This fundamental discovery opens up an entirely new field of research and reveals some of the mysteries behind the unfamiliar auditory functions," said Barbara Canlon, professor of auditory physiology at the Department of Physiology and Pharmacology at Karolinska Institutet.

The study was published in the journal Current Biology.


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