It was investigated whether cognitive performance shows a circadian rhythm during a 50 h-long forced desynchrony sleep-wake-schedule. We asked whether it would be possible to estimate the circadian period of cognitive performance under such circumstances and how strong it correlates to subjective sleepiness rating as well as body temperature.
Cognitive performance was evaluated using the Bowles-Langley test (BLT), which estimates cognitive performance by capturing reaction time and counting errors in a 2-choice visual search task.
Power spectral densities (PSD) were estimated by the Lomb-Scargle periodogram . The circadian period τc was estimated from peak PSD. PSD of BLT scores showed lowest, yet distinct, circadian amplitude.
In addition to the circadian period estimation we analyzed the correlation of the acquired variables against each other. Pearson’s correlation coefficients were significant but varied strongly at a commonly low level.
Despite the obstacle of a plethora of influencing factors  BLT scores are sensitive to the circadian rhythm and provide correct estimates of τc compared to rectal body temperature (RBT) as reference. Subjective measures failed estimating τc.