An experiment was carried out to investigate the hypothesis that task difficulty is reflected in changes in the topographical distribution of the ongoing EEG. Subjects had to perform three different tasks at two difficulty levels each; the Sternberg memory scanning task in an auditory and in a visual mode and a task whose performance required mainly visual scanning. Task difficulty was verified by the measurement of response times. Using a commercial Brain Electrical Activity Mapping device, EEG was recorded from 19 scalp electrodes while the subjects performed the tasks. Spectral matrices of the EEG were calculated to investigate spatial relationships in the EEG. Compared to the lower level, higher task difficulty resulted in EEG changes that led to the identification of two factors. One was the reduction of parietal and occipital alpha activity due to the amount of visual scanning and the other an increase of theta activity in the left frontal electrodes which may be associated with the amount of general mental processing.

SUMMARY Numerous anecdotes in the past suggest the concept that sleep disturbances in astronauts occur more frequently during spaceflight than on ground. Such disturbances may be caused in part by exogenous factors, but also an altered physiological state under microgravity may add to reducing sleep quality in a spacecraft. The present investigation aims at a better understanding of possible sleep disturbances under microgravity. For the first time, experiments were conducted in which sleep and circadian regulation could be simultaneously assessed in space. Four astronauts took part in this study aboard the Russian MIR station. Sleep was recorded polygraphically on tape together with body temperature. For a comparison, the same parameters were measured during baseline periods preceding the flights. The circadian phase of body temperature was found to be delayed by about 2 h in space compared with baseline data. A free-run was not observed during the first 30 d in space. Sleep was shorter and more disturbed than on earth. In addition, the structure of sleep was significantly altered. In space, the latency to the first REM episode was shorter, and slow-wave sleep was redistributed from the first to the second sleep cycle. Several mechanisms may be responsible for these alterations in sleep regulation and circadian phase. Most likely, altered circadian zeitgebers on MIR and a deficiency in the process S of Borbély's sleep model cause the observed findings. The change in process S may be related to changes in physical activity as a result of weightlessness.

The hormone melatonin is currently proposed by some investigators to be an efficient means for decreasing the impairing effects of jet lag. Eight healthy male subjects, aged 20 to 32, underwent a 9-hr advance shift in the isolation facility of our institute during two periods each of 15 days' duration. In a double-blind, crossover design, subjects took either melatonin or placebo at 1800 hr local time for 3 days before the time shift and at 1400 hr for 4 days afterwards. The time shift was simulated on days 7 and 8 by shortening the sleep period by 6 hr and the following wake period by 3 hr. Body temperature was recorded every 90 min, and urine was collected at 3-hr intervals all day and night. Melatonin treatment enhanced the resynchronization speed of some, but not all, hormone and electrolyte excretion rates for several days after the time shift. The adaptation speed of the temperature rhythm significantly increased during one postshift day. In addition, the circadian temperature rhythm had a significantly higher amplitude under melatonin treatment than under placebo after the time displacement. For the placebo group, the rhythm of 6-hydroxymelatoninsulfate excretion exhibited an advance shift in five subjects, whereas the other three showed a delay shift, and adjustment did not achieve more than one-half of the expected value within 8 days. A significantly different adjustment could be observed in the melatonin-treated group: Seven subjects underwent an advance shift of the expected 9 hr within an average of 8 days. The results suggest that melatonin treatment can accelerate resynchronization of the melatonin excretion rhythm after eastward time zone transitions. The improvement is not, however, sufficiently great that we can recommend melatonin for the alleviation of jet lag.

SUMMAR Y The aim of the present study was to evaluate time-on-task effects on subjective fatigue in two different tasks of varying monotony during night-time testing (20:00 to 4:00 hours) in a sleep deprivation intervention. The experiment included eight test runs separated by breaks of approximately 20 min. Twenty healthy volunteers performed a driving simulator and the Mackworth clock vigilance task in four of the test runs each. Sequence of tasks was varied across subjects. Before and after each task, subjective sleepiness was assessed by means of the Karolinska sleepiness scale and subjective fatigue was rated on the Samn-Perelli checklist. Fatigue and sleepiness significantly increased over the course of the night. Both tasks led to an increase in fatigue and sleepiness across test runs. However, this time-on-task effect was larger in the vigilance than in the driving simulator task. It is important to note that fatigue and sleepiness in one test run were not influenced by the task performed in the preceding test run, that is there were no cross-over effects. The results suggest that time-on-task effects superimpose circadian and sleep-related factors affecting fatigue. They depend on the monotony of the task and can be quantified by means of a design including separate test runs divided by breaks.k e y w o r d s driving simulator, fatigue, model, time-on-task, vigilance