Circadian temporal adaptation and the perception of time

As in other vertebrates, the human circadian system is characterized by a distinct temporal order of its components. This order is maintained by the coupling forces between various oscillators as well as by the entraining signals from the Zeitgebers (see Chapter 12). There is hardly a tissue or function that has not been shown to have some 24-hr variation. As an example, Figure 1 presents results of an experiment in which six subjects were held in groups of two on the same rigorous schedule. Although there were considerable interindi-vidual differences and also day-to-day variations, patterns like those shown in Figure 1 are satisfactorily reproducible. This reproducibility is again illustrated in Figure 2, which summarizes data on plasma Cortisol collected in six laboratories. To account for possible phase-controlling effects of sleep (see below), the curves are normalized with regard to the various sleep times of the subjects. Despite the fact that Cortisol, like many other hormones, is secreted in a highly variable sequence of episodes (see Chapter 12, Figure 1), a clear circadian pattern emerges in each curve, averaged from the data on n subjects, and there is perfect correspondence in phase and relative amplitude between the curves. It is noteworthy that two submaxima appear in all curves at about the same circadian phases. Presumably, they are not consequences of the usual meal timing but may represent a rhythm component interposed between the circadian domain and the episodes (cf. the discussion in Aschoff, 1979).

The Circadian System of Man

Handbook of behavioral neurobiology. Vol. 4. Biological rhythms: Jürgen Aschoff. Plenum Press, New York, 1981. ISBN 0-306-40585-7. Price not given.

The relative effectiveness of external zeitgebers synchronizing circadian rhythms can be evaluated by mesuring the size of the range of entrainment. The experimental approach to measure entrainment limits is the application of an artificial zeitgeber with slowly and steadily changing period. In human circadian rhythms, an absolute light-dark (LD) cycle with a light intensity during L of 100 lux or less, results in an upper entrainment limit of 26.91±0.24 hours. The same limit is reached in constant illumination when only informations are given to the subjects. Consequently, the LD cycle is effective mainly with its behavioral component characterized by the request of the light-dark alternation to go to rest. In experiments with the same experimental protocol but higher intensity of illumination during L (∼400 lux, i.e., exceeding the threshold beyond which melatonin excretion is suppressed in humans), human circadian rhythms can be synchronized within a much larger range; the upper entrainment limit is, with all overt rhythms measured, beyond 29 hours. This means that bright light has an effect on the human circadian system which is qualitatively different from that of dim light, and which is similar to the effect of light in most animal experiments. This finding has theoretical and practical implications.

Bright light affects human circadian rhythms.

1 Introduction.- 1.1 The Search for Principles of Physiological Organization in Vertebrate Circadian Systems.- 1.2 Zeitgebers, Entrainment, and Masking: Some Unsettled Questions.- 2 Pathways of Zeitgeber Signals.- 2.1 Comparative Aspects of Retinal and Extraretinal Photosensory Input Channels Entraining Endogenous Rhythms.- 2.2 Neuroanatomical Pattern of Endocrine and Oscillatory Systems of the Brain: Retrospect and Prospect.- 2.3 Unspecific Optic Fibres and Their Terminal Fields.- 2.4 Characteristics of the Interaction Between the Central Circadian Mechanism and the Retina in Rabbits.- 3 The Role of the Nucleus Suprachiasmaticus.- 3.1 Physiological Models of the Rodent Circadian System.- 3.2 Neuroanatomical Organization and Connections of the Suprachiasmatic Nucleus.- 3.3 CNS Structures Controlling Circadian Neuroendocrine and Activity Rhythms in Rats.- 3.4 The Neurophysiology of the Mammalian Suprachiasmatic Nucleus and Its Visual Afferents.- 3.5 Neurophysiological Studies of the SCN in the Rat and in the Java Sparrow.- 3.6 Neural Mechanisms in Avian Circadian Systems: Hypothalamic Pacemaking Systems.- 3.7 Limits of Entrainment to Periodic Feeding in Rats with Suprachiasmatic Lesions.- 4 The Role of the Pineal Organ.- 4.1 Phase Responses and Characteristics of Free-Running Activity Rhythms in the Golden Hamster: Independence of the Pineal Gland.- 4.2 Circadian and Infradian Activity Rhythms in the Mammalian Pineal Body.- 4.3 Electrophysiology of the Mammalian Pineal Gland: Evidence for Rhythmical and Non-Rhythmical Elements and for Magnetic Influence on Electrical Activity.- 4.4 Circadian Rhythms of the Isolated Chicken Pineal in Vitro.- 4.5 Endogenous Oscillator and Photoreceptor for Serotonin N-Acetyltransferase Rhythm in Chicken Pineal Gland.- 5 Systemic Aspects.- 5.1 Role of Hormones in the Circadian Organization of Vertebrates.- 5.2 The Neuropharmacology of Circadian Timekeeping in Mammals.- 5.3 Entraining Agents for the Circadian Adrenocortical Rhythm in the Rat.- 5.4 Splitting of the Circadian Rhythm of Activity in Hamsters.- 5.5 Phase-Response Curves and the Dual-Oscillator Model of Orcadian Pacemakers.- 5.6 Orcadian Control of Body Temperature in Primates.- 6 The Circadian Sleep-Wake Cycle.- 6.1 Circadian and Sleep-Dependent Processes in Sleep Regulation.- 6.2 Sleep Circadian Rhythms in the Rat: One or Two Clocks?.- 6.3 Sleep ECoG Rhythm in the High Mesencephalic Rat.- 6.4 Interaction Between the Sleep-Wake Cycle and the Rhythm of Rectal Temperature.- 6.5 The Phase-Shift Model of Spontaneous Internal Desynchronization in Humans.- 7 Photoperiodic Phenomena Related to the Circadian System.- 7.1 Physiology of Photoperiodic Time-Measurement.- 7.2 Pineal Influences on Circannual Cycles in European Starlings: Effects Through the Circadian System?.- 7.3 Complex Control of the Circadian Rhythm in N-Acetyltransferase Activity in the Rat Pineal Gland.- 7.4 The Critical Photoperiod in the Djungarian Hamster Phodopus sungorus.- 8 General and Functional Aspects.- 8.1 Circadian Contributions to Survival.- 8.2 Daily Temporal Organization of Metabolism in Small Mammals: Adaptation and Diversity.- 8.3 Characteristics and Variability in Entrainment of Circadian Rhythms to Light in Diurnal Rodents.- 8.4 Functional Significance of Daily Cycles in Sexual Behavior of the Male Golden Hamster.

Vertebrate circadian systems : structure and physiology

UNLABELLED In an underground isolation unit, 42 subjects were living singly for time spans of at least 7 days up to more than a month. Except three who were entrained to 24 h by an externally controlled light-dark cycle (LD), subjects had no time cues and developed free-running circadian rhythms either in self-selected LD-cycles or in constant illumination. Each subject had to give a signal whenever he thought that 1 h had passed. In addition, 30 subjects produced short-time intervals within the range from 10 to 120 s. RESULTS The 1-h estimates were longer than 1 h, and had a strong positive correlation with the duration of wakefulness alpha as well as with the length of the circadian cycle. The short time estimates were equally distributed between under- and over-estimation of the required interval, and they were neither correlated with the 1-h estimates nor with alpha. It is concluded that long and short time estimates are based on different mechanisms.

Circadian temporal adaptation and the perception of time

References

The Circadian System of Man

Handbook of behavioral neurobiology. Vol. 4. Biological rhythms: Jürgen Aschoff. Plenum Press, New York, 1981. ISBN 0-306-40585-7. Price not given.

Bright light affects human circadian rhythms.

Vertebrate circadian systems : structure and physiology

On the perception of time during prolonged temporal isolation.

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