Yesterday, I showed how environmental, behavioral, and social cues act as zeitgebers to human circadian rhythms, and I tried to be as thorough as possible (without outstaying my welcome). I left out one very important environmental cue with the promise of more information today – sound.

I can’t recall exactly where I heard about it, but it was someone’s offhand reference to the notion of the calls of songbirds affecting our circadian rhythms that convinced me I should do a follow-up to the blue light piece from earlier this year. The notion of bird calls affecting us on a deeper level than a regular sound makes some intuitive sense to me, so I did some searching to see if there was anything to it.

There’s certainly a precedent for the phenomenon in animal research. For years, researchers have known that auditory cues exert potent entraining effects on the circadian rhythms of animals. Sounds from a “large animal colony where there were many cats and people,” for example, entrained the circadian rhythms of isolated domestic cats, while common house sparrows’ circadian locomotor rhythms were entrained by the playback of pre-recorded bird song. In squirrel monkeys, the zeitgeber effect of sound cues becomes pronounced when the master pacemaker is partially disabled. So the mechanism clearly exists in certain mammals and birds, but what about humans?

I was only able to find one study, but it’s a good one, and the full text is free. Take a look for yourself.

Ten subjects spent two 4-day lab sessions in constant dim (read: not a significant source of photic entrainment) light. Between 1 AM and 3 AM, before the core body temperature minimum was reached, on the second and third nights of each session, half were subjected to a 2-hour CD of bird calls set to classical music at an intensity equivalent to normal human conversation (this wasn’t blaring. overwhelming noise); the control half were subject to a blank CD they were told contained an auditory stimulus “above the human hearing range.”

Both groups experienced delays in dim light melatonin onset (DLMO) – “a reliable marker of circadian rhythm phase” – and core body temperature (CBT) phases, with the group receiving the auditory stimulus experiencing significantly greater delays. Self-reported feelings of fatigue were also lower in the auditory stimulus group. These results mark the first (and, to my knowledge, only) recorded incidence of auditory cues stimulating circadian phase shifts in humans, “with shifts similar in size and direction to those of other non-photic stimuli” more commonly researched.

Light is the granddaddy of zeitgebers, consistently producing the most amplified phase shifts when compared to non-photic stimuli, but it’s notable that the study’s auditory stimulus produced phase delays that were “consistent in direction with those produced by light.” That is, light input has been shown to induce phase delays (lowered melatonin, delayed “sleepiness” when presented before CBT minimum and phase advances (increased melatonin, advanced “sleepiness”) when presented after CBT minimum. While this study only tested the effect of bird song on circadian phases before CBT minimum, its similarities with light input suggest that bird song might advance the circadian phase if presented post-CBT minimum. That wasn’t tested, though, so we don’t yet know for sure.

Furthermore, most circadian studies of light input employ longer and/or more frequent light pulse durations when compared to the sound study, which may explain the difference in phase shift amplitude. In fact, when you compare this study to studies that use light in similar amounts, the amplitude of circadian phase delay is “of similar magnitude.” It appears as if sound cues have great potential as circadian zeitgebers.

Another question the study’s author raises is whether the source of the sound matters – would we see the same effect on circadian phases if the subjects were presented with a CD playing the soothing sounds of rush hour traffic? Or what if you dropped the classical and kept the bird songs, or vice versa? Is it the mere presentation of auditory stimuli – any auditory stimuli – that matters? Would a two hour conversation with someone else do the trick?

An interesting question, eh? For me at least, it’s not difficult to imagine that specific sounds – like those we would have heard over tens of thousands of years of evolution – may have more of an effect on our millions-of-years-in-the-making circadian rhythms than random, more evolutionarily novel noises. Note that in previous animal studies, researchers were sure to use auditory stimuli that “made sense” to the animals in question: cat sounds for cats and bird songs for birds. As anyone who’s woken up in nature (camping in the mountains, maybe, or deep in the rainforest) can attest, wildlife tends to erect a wall of sound, especially in the morning. You can’t escape it, and any early human ancestors living in the wild would surely have grown up knowing, hearing, and experiencing the sound of birds and other animals on a daily basis.

I mean, we don’t “just wake up.” Hormones, regulated by the circadian clock and entrained by light, sound, and other cues, wake us up.

I don’t usually use an alarm clock. For those rare occasions when I do and as a test, I recently downloaded a new bird chirp ring tone to use as my morning alarm clock instead of the annoying stock tone. I haven’t taken any body temperature or melatonin phase shift measurements, but it certainly is pleasant to wake up to the sound of bird song. This makes me wonder about alarm clocks in general. What’s really happening when we’re awoken by the alarm? I know that the basic shock of a sudden noise (any noise) is what wakes us up initially, but what if a “soothing,” more evolutionarily-congruent sound can make that wake up experience more pleasant and less jarring by shifting our circadian phase, too? Does morning light wake us up through brute, mechanical force, or does it tug at our circadian pacemaker, urging it to produce more cortisol and less melatonin?

This is exciting stuff. Nerdy stuff, sure, but exciting. And, of course, speculative. I’m going to stick with the bird song wake up call and see where it takes me. If anyone else gives it a try (maybe with real birds), let me know how it turns out.

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