Circadian Rhythm and Hearing
Circadian rhythms represent physical, mental, and behavioral changes that follow a roughly 24-hour cycle. The master clock that controls circadian rhythms is called the suprachiasmatic nucleus (SCN). The circadian rhythm is endogenous, but also adjusted or entrained by local environmental cues (called zeitgebers, German for “time giver”); which include light, temperature, and redox cycles.
Circadian regulation can also influence auditory function as reviewed recently by Basinou et al. (2016). Auditory cues can act as zeitgebers and alter circadian rhythms in mammals including humans. Also, studies in rodents display greater acoustic startle response in the morning (~9 AM) than in the evening (~9 PM). However, of even greater interest is that sensitivity to noise trauma may also differ based time of day of exposure. Mice exposed to a moderate noise (100 dB SPL for 1 hour) showed comparable ABR threshold shift at 24 hours whether exposed in the morning or at night. However, at two weeks, the animals exposed during the day demonstrated complete recovery of thresholds, while animals exposed at night showed a significant permanent threshold shift. Studies in humans also support variability in cochlear function related to circadian influence as measured by otoacoustic emissions.
The impact of circadian rhythms and susceptibility to acquired hearing loss is a major factor for hearing conservation, particularly in humans with circadian disruptions such as shift workers and military personnel. If night shift workers are at greater risk for hearing loss, we may need to adjust regulations for noise dose based on work schedule. However, what remains unknown is how entrainment can influence this risk? In other words, does the system adjust to the new timetable mitigating risk? Basinou et al. provide an excellent overview of the topic and some of the proposed mechanisms involved.
Basinou V, Park, J-s, Cederroth CR, Canlon B. (2016) Circadian regulation of auditory function, Hearing Research August 18.