A clock need not have hands or a digital display. A recurring biological process can supply a phase: a position within a cycle that helps coordinate other processes. But not every daily pattern demonstrates an internal clock. A system might simply respond whenever the environment changes.
Response or oscillator?
Imagine a device that activates only when a lamp is switched on. If the lamp follows a daily schedule, the device shows a daily pattern. Yet that observation alone does not reveal whether the device can generate a cycle of its own.
Now imagine a system that continues to cycle when the daily cue is removed under otherwise suitable constant conditions. Its continuing rhythm supplies evidence for an internal timing process. The period may differ from precisely twenty-four hours.
This distinction separates an environmental schedule from a system's capacity to sustain an oscillation. It also suggests an experimental question: what continues, what drifts, and what stops when the recurring external cue is changed?
A period that drifts
Take an invented oscillator with a twenty-five-hour period and compare its recurring peaks with a twenty-four-hour reference. Begin with a shared peak at hour zero.
| Cycle number | Daily reference peak | Oscillator peak | Delay |
|---|---|---|---|
| 0 | 0 h | 0 h | 0 h |
| 1 | 24 h | 25 h | 1 h |
| 2 | 48 h | 50 h | 2 h |
| 3 | 72 h | 75 h | 3 h |
The delay increases because the periods differ. There is no need to say that physical time itself has slowed inside the oscillator. Both schedules are being compared using the same time unit.
The example also shows why “daily” can be ambiguous. It might mean a rhythm whose period is approximately one day, or a rhythm kept in a stable relationship with the environmental day. Those descriptions need not coincide.
What experiments reveal
Konopka and Benzer's 1971 study of fruit flies identified mutants with markedly different circadian patterns: approximately nineteen-hour and twenty-eight-hour periods, and an arrhythmic form. The work connected genetic differences with altered timing in the measured behaviours. It made the clock problem experimentally accessible through changes in the organism.
A different approach isolated a molecular timing system. Nakajima and colleagues reconstituted sustained oscillations in KaiC phosphorylation using the cyanobacterial proteins KaiA, KaiB and KaiC together with ATP. Their 2005 study reported temperature compensation and correspondence between periods in selected mutant systems in vitro and in vivo.
The two studies approach timing at different levels. One follows organismal rhythms and genetic changes; the other examines a biochemical oscillation in a prepared system. Neither requires us to treat a “clock” as a separate little object commanding the rest of biology. The timing can be a property of interacting processes.
Entrainment and phase
An oscillator can interact with recurring environmental signals. When the interaction produces a stable timing relationship with the external cycle, we speak of entrainment. The important feature is the maintained phase relationship, rather than the mere observation that two average periods look similar.
In our illustrative example, entrainment to a daily cue would prevent the unchecked one-hour-per-cycle drift. How that happens depends on the oscillator and on when and how the cue acts. A full model must specify that response; the word “synchronization” is not itself a mechanism.
There is also a difference between counting cycles and knowing where one is inside a cycle. A count supplies an accumulated order; phase distinguishes stages between successive peaks. Biological timing can involve both coordination within a cycle and relations across many cycles.
What “internal” means here
An internal biological clock is internal to an organism or molecular system in an operational sense: the system sustains part of the timing process. It still exchanges energy and can respond to environmental conditions.
This use of “internal” should be distinguished from the proposed inner generation discussed in Duality of Time. Biological clocks operate within the physical time measured by laboratory instruments. A deeper account would need to explain those instruments and biological processes together, while recovering their experimentally observed relations.
The connection is a question of coordination: how do recurring processes create stable, usable distinctions? Begin with what a measured rhythm actually establishes, and then ask what a broader model adds.
Sources and further reading
- Konopka and Benzer (1971), “Clock mutants of Drosophila melanogaster,” PNAS 68, 2112–2116, DOI; Caltech record and paper.
- Nakajima et al. (2005), “Reconstitution of circadian oscillation of cyanobacterial KaiC phosphorylation in vitro,” Science 308, 414–415, DOI; author-institution record.
- Why durations feel different: biological rhythms and reported duration are different questions about temporal experience.