The sensitivity of postural responses to the effects of work fatigue and circadian changes was explored in a pilot study using a specific method of multiplate posturography based on the differential assessment of vertical pressure on four separate platforms for each heel and set of toes of each foot, respectively 8 normal subjects, M.D.s, were given six posturographic examinations immediately before and after three 8-hr. work shifts in the emergency room of a major hospital in Tel-Aviv. 3 posturographic measures (stability, spectral power of postural sway at low frequency of 0.1-0.25 Hz, and unilateral weakening of heel-toe synchronisation) were significantly affected by work fatigue whilst also showing interaction with circadian rhythm. 2 additional measures (power of sway at high frequency of 1.00-3.00 Hz and dysharmonic distribution of weight over the four platforms) were not related to workload but showed significant circadian changes. These effects appeared only on positions involving restricted visual and somatosensory feedback causing vestibular stress. Results justify the application of multiplate posturography as an ancillary tool in measuring objectively the effects of fatigue and circadian changes as well as the interaction between endogenous chronobiological processes and their external conditioning factors (Zeitgebers). Pragmatic implications of the findings in the context of industrial medicine and interdisciplinary efforts to prevent road and air accidents are discussed.

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Studies suggest some physiologic, cognitive, and behavioral 24h rhythms are generated by cyclic components that are shorter in period than circadian. The aim of this study was (1) to examine the hypothesis that 24h human performance rhythms arise from the integration of high-frequency endogenous components and (2) to quantify the contribution of each higher frequency component to the phenotype of the rhythm. We monitored the performance of 9 experienced pilots by employing an array of cognitive-based tests conducted in a flight simulator so that, over the 6-day experiment, data were obtained for each 2h interval of the 24h. The activity-rest schedule of the subjects, no matter the exact clock time schedule of sleep and activity, always consisted of 14h activity (when they carried out regular professional duties) and 10h rest, with at least 8h of sleep. The simulated combat scenarios consisted of simple and complex tasks associated with target interception, aircraft maneuvering, and target shooting and downing. The results yielded two indices: the number of prominent periodicities in the time series and the relative magnitude of the amplitude of each relative to the construction of the composite 24h waveform. Three cyclic components (8h, 12h, and 24h) composed the observed 24h performance pattern. The dominant period and acrophase (peak time) of the compound output rhythm were determined by the interplay between the amplitudes of the various individual ultradian components. Task complexity (workload) increases the expression of the ultradian entities in the 24h pattern. We constructed a model composed of the multiple ultradian components; the composite output defined a "time span" (of 2h-4h duration) as opposed to an exact "time point" of high and low performance, endowing elevated functional capability.

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