There are circadian (approximately 24 h) rhythms for a wide range of human physiological and psychological functions including mood and performance efficiency. These rhythms are self-sustaining in conditions of temporal isolation, indicating that internal oscillators (or biological clocks) control them. Recent research has proposed an endogenous two-oscillator model of the human circadian system, with one oscillator indicated by the core body temperature rhythm and a second oscillator responsible for the daily cycle of sleep and wakefulness. The present study was designed to produce a desynchronization in period between the two oscillators, to assess directly the behaviour of the rhythms of different performance tasks. The results, reported here, indicate that a simple manual dexterity task is almost entirely under the control of the temperature rhythm oscillator, whereas a more complex cognitive task demonstrates a periodicity which appears to be influenced by those oscillators controlling temperature and the sleep/wake cycle.

It is well established that there is a complex timekeeping mechanism in the human brain. This mechanism is associated with a variety of physiological and psychological rhythms having a period of about a day, and thus referred to as circadian rhythms. The circadian system has recently been modeled in terms of two underlying oscillators, one much more resistant to changes in routine than the other. These oscillators are considered to be endogenous, that is, internal to the organism, and not reliant for their existence upon changes in the person's environment or general behavior. They thus continue to run even when the sleep/wake cycle is suspended, as in sustained operations. Thus, by their very nature, sustained operations require the individual to override the inputs that are coming from his or her circadian system (especially the indication that sleep is required). The aim of this paper is to provide a background to the area of circadian rhythms research, including a section on the methodology, so that the impact of the circadian system on sustained operations can be better understood. This paper has two purposes. The first is to discuss recent findings in the area of human circadian (approximately 24-h) rhythms, and their implication for sustained operations. The second is to describe the methodology that is used to study these rhythms in the laboratory, and uses as an example the particular techniques employed at the Instituteof Chronobiology, Cornell University Medical College.

In order to document circadian rhythmicity in various psychological functions under the chronobiologically 'pure' condition of temporal isolation, a battery of mood and performance tests were administered about 6 times per day to a heterogeneous group of 18 subjects (ages 19-81, 5 female). Each subject spent about 5 days in temporal isolation, entrained to a routine equivalent to his/her own habitual sleep/wake cycle. Average time of day functions were obtained for the mood and performance variables, and compared to rectal temperature data subjected to exactly the same statistical analysis. Significant time of day effects were found in the mood variables of alertness, sleepiness, weariness, effort required, happiness and well-being. Times of 'best' mood were different from the time of peak temperature. Moreover, the minima of sleepiness, weariness and effort occurred earlier in the day than the maximum of alertness. Significant time of day effects were also found in the speed with which search and dexterity tasks were completed. Only the dexterity tasks showed a complete parallelism with the temperature rhythm.