Evolutionary Simulations to Determine the Human Circadian Period Using an Extended Sleep-Wake Model

The development of the human sleep-wake cycle and the adaptation to the changing day and night condition on our planet took place over a time frame of 100000 years. As the result, a representative cross section of today´s population sleeps between 4 and 11 hours, with a midsleep point between 1.00 and 9.00 am and a period length of the human circadian pacemaker between 23.5 and 25 hours. Roenneberg et al. published the distribution of midsleep point and sleep duration based on an extensive questionnaire which represents the Middle European society. Czeisler presented a normal distribution of the period length of the human circadian pacemaker as result of a forced desynchrony study. The sleep wake characteristic can be described with the well established two-process model (2PM). The adaptation of the period of our pacemaker to the 24-h day by light is best understood in terms of the phase response curve (PRC). We introduce a combination of these two well established models, called extended two-process model (E2PM). With this model, the sleep-wake behaviour and the circadian period of man under natural day (light) and night (dark) conditions can be simulated simultaneously. With this model, 250 different sleep-wake types were parametrized using evolutionary algorithms. As a breakthrough, the resulting distribution of one important parameter, the sleep-wake cycle duration, matches closely the experimentally acquired distribution of Czeisler.