Circadian rhythms and patterns of performance before and after simulated jet lag

We have developed a rhesus monkey model that enables us to investigate physiological rhythms and circadian effects on performance in an integrated framework. Monkeys worked for 8 h/day on a two-component task (a vigilance trial followed by a discrimination trial) for their daily food aliquot. Concurrently, we recorded activity and temperature rhythms around the clock. To test the model, we studied rhythms and performance during entrainment to a 24-h light cycle and after a 6-h phase advance. Results from this animal model displayed many of the essential characteristics seen in similar human experiments. During stable entrainment, temperature rhythms reached their maximum amplitude in late afternoon, with activity rhythms reaching their maximum amplitude several hours earlier. Performance exhibited consistent task-dependent variations over the course of daily sessions. Speed of discrimination performance was fastest at the beginning of the session, and speed of vigilance performance was fastest several hours later. After a 6-h phase advance, monkeys exhibited transient internal desynchrony with activity resynchronizing faster than temperature. Both vigilance and discrimination were impaired after the phase shift, with vigilance exhibiting larger-magnitude and longer-lasting impairments than discrimination. A second drop in performance was seen 10-14 days after the phase shift. These data replicate and extend earlier work in humans and show that this model can be used in the study of chronobiological questions that would be too expensive or too impractical to do with humans.