William Meissner notes that when the diurnal rhythm of the nervous system is examined, experiments can be used which deprive us of external prompts regarding time. Such prompts are categorised into time-sources which are artificial (such as clocks), versus those which are natural (such as sunlight).

It should be noted that all biological systems evidence biological rhythms, most of which are patterned on a diurnal basis (Moore-Ede et al. 1982). The human organism is no exception to this principle (Wever 1979). Even in man there exist a variety of diurnal rhythms. The more obvious rhythmic systems include the cardiovascular system, the cardiorespiratory system, renal system, gastrointestinal system, endocrine systems, changes in blood constituents, patterns of activity in the autonomic nervous system, and variations in mood and performance. Also rhythmic patterns in the neuromotor system are significant for regulation of walking, running, etc. Rhythmic activity has also been identified in the nervous system as a whole. As Gooddy (1969) commented, “If we accept the notion of the nervous system as a clock form, we note immediately the complex nature of its structure. The final clock, by which perhaps we say ‘we know what time it is,’ or ‘we know about time and its passing,’ is the last abstraction from the innumerable subsidiary clock forms. Even at a level of single brain cells, rhythms have been demonstrated by Phillips (1956). And the common clinical tool of the electroencephalogram (EEG) provides us with objective evidence of summated and abstracted rhythmic nervous activity” (248-49). The inherent diurnal rhythmicity of the nervous system was established in a series of experiments depriving subjects of all environmental time cues-without the aid of any artificial time aids (e.g., wristwatches) or natural time-related phenomena (e.g., light-dark sequences), diurnal rhythms were found to persist throughout lengthy periods lasting as long as several months (Gifford 1981; van Cauter and Turek 1986).

Investigators found that these free-running rhythms did not coincide exactly with a twenty-four-hour cycle, but varied among individuals, approximating a circadian pattern but tending in humans to extend the cycle slightly beyond the twenty-four-hour measure. When exposed to environmental cues, however, these rhythms tend to resynchronize, more or less, with the normal twenty-four-hour cycle. This pattern of temporal organization undoubtedly has adaptive evolutionary advantages and serves the interests of internal organization and synchronization of functions, both physiologically and psychologically. We can conclude that such synchronizations are an aspect of normal and healthy functioning, and that pathological dysfunctions can reflect disruption in these systems. Experiments in sleep deprivation and common phenomena such as jet lag seem to reflect this understanding. The incidence of health complaints is higher among night-shift workers than day workers, presumably because of the desynchronization between normal restactivity cycles and environmental time cues (Moore-Ede et al. 1982) (Meissner 2007, 225-26).

Meissner, William. 2007. Time, Self, and Psychoanalysis. Lanham: Jason Aronson.