Rhythm and Frequency in Nature, Body and Life

Two words, one principle

Frequency refers to how often a periodic process repeats per unit of time; one hertz is one repetition per second. In a 440 Hz tone, air pressure and the movement of air particles at a fixed location fluctuate 440 times per second. Human hearing perceives something roughly between 20 Hz and 20,000 Hz, although the upper limit slowly declines from young adulthood onwards.

Frequency can describe any rate of repetition. With slowly successive sound events, we perceive individual pulses and their temporal order as rhythm; with rapid periodic repetition, a continuous sound with pitch can arise. The transition is gradual and depends on the signal: the auditory impression often changes in the range of approximately 20 to 30 repetitions per second from individual pulses or fluttering to very low pitch.

Outside

The Earth's rotation produces the alternation of day and night. Because the axis of rotation is inclined by around 23.4 degrees to the plane of Earth's orbit, the position of the sun and the length of the day change over the course of the year; this produces the seasons. Many plants use day length together with their internal clock as a seasonal signal for flowering. Depending on the species, however, ambient temperature, a prolonged cold period, age, hormones and further conditions also play a role; day-neutral plants do not flower because of day length. The time of flowering affects growth and reproductive success, but which time is favourable depends on the species and the environment.

The moon as a second rhythm

An average lunar day lasts around 24 hours and 50 minutes, because the Earth has to rotate a little further beneath the moving moon to catch up with it again. On most coasts, there are two high and two low tides per lunar day; in an idealised semidiurnal rhythm, consecutive high tides are around 12 hours and 25 minutes apart. Coastal shape and sea basins, however, produce semidiurnal, mixed or even diurnal patterns, which makes local tide tables crucial. On average, lunar-related times shift by around 50 minutes each day.

Inside

In carefully controlled laboratory protocols, the intrinsic period of the human day-night clock averaged 24.18 hours. This approximately daily, or circadian, rhythm is repeatedly aligned with environmental time by light. The central clock is the suprachiasmatic nucleus in the hypothalamus. Evening or night-time light can – depending on intensity, spectrum, duration, biological timing and the person – suppress melatonin and shift the internal phase. Such effects have already been measured after exposures of around 30 to 60 minutes; their strength varies considerably.

The rhythms beneath the daily clock

Sleep is not a uniform surface: A passage through non-REM sleep (sleep without the rapid eye movements typical of REM) and REM sleep typically lasts around 80 to 100 minutes; there are usually four to six cycles per night. Deep sleep is concentrated more at the beginning, REM sleep more in the later night. In the fasting state, the stomach and small intestine pass through recurring phases of activity, the migrating motor complex. A cycle is often roughly in the range of 80 to 120 minutes, but varies. The strong phase III can begin in the stomach or the duodenum and travels further; eating interrupts this fasting pattern. A phase III beginning in the stomach can coincide with stomach growling and an increase in hunger, but not every abdominal sound comes from it. The basic electrical activity of the human stomach, the slow wave, is around three cycles per minute. It provides the temporal framework for possible contractions, but not every wave produces a contraction.

Heart and breathing are two coupled rhythms. At rest, the heart typically becomes faster during inhalation and slower during exhalation. In mammals, this breathing-related pattern is called respiratory sinus arrhythmia. Similar cardiorespiratory couplings are found in all studied groups of air-breathing vertebrates, although their expression and function differ. Mediation through the vagus nerve is central. The strength of the fluctuation is often used as an indirect indication of cardiac vagal regulation, but also depends on breathing depth and pace, posture, age and further factors. In acute stress, vagally influenced values decline in many studies, but not without exception. This is an observation, not a prescription; the music is not presented here as a treatment.

The menstrual cycle is also a biological rhythm whose length can vary between people and in the same person. The figure of 28 days is a simplification. In a large analysis of 612,613 ovulatory cycles logged by app, the average was 29.3 days; this describes this sample of users and not the target value for every person. FIGO refers to intervals of 24 to 38 days as normal menstrual frequency in 18- to 45-year-olds. “Regular” additionally means that the cycle lengths of one person differ from each other only to a limited extent; the permissible range depends on age. An average therefore describes individual variation only incompletely. Biological rhythms, too, are rarely metronomically exact.

Where science ends

What has been described so far is physiology and astronomy, measured and verifiable by calculation. Two other levels must be distinguished from it.

Where model and tradition begin

Model assumptions: The phrase “everything vibrates” is very powerful as an image and very broad as a statement. That rhythms exist in the body is documented. That all these rhythms form a common order that could be addressed from outside is an assumption, not a finding. I consider it worthwhile. I do not claim that it has been proven.

Tradition and spirituality: Musical and contemplative traditions have known for centuries that sound has to do with breath, pulse and attention, and I work with this knowledge. It is experiential knowledge. It has a different status from a series of measurements, and it should not pretend to have the same one.

That the auditory system can respond to periodic stimuli with measurable temporal responses is documented. Whether and how this produces changes in other bodily rhythms or a desired experience depends on the stimulus, the person and the situation and is only partly understood. When composing, I work with this open question.