Circadian parameters as individual characteristics.

Human circadian rhythms that are free running in temporal isolation can show an extreme variability of the sleep-wake cycle without the subject being aware of it. This is due mainly to a systematic adjustment of certain circadian time structures to the varying length of the "day": the perception of long time intervals in the range of 1 h and the intervals between meals have a strong positive correlation with the duration of wake time alpha, in contrast to the amount of hourly locomotor activity that is negatively correlated with alpha. So far, the dependence of locomotor activity, of the "1-h" intervals, and of the intermeal intervals on alpha is well documented for the data sets of single subjects. The present article demonstrates that the correlations found intraindividually also apply if data from several subjects are compared. If plotted as a function of the mean individual alpha, the individual means of "1-h" intervals and of the intermeal intervals show the same positive correlation and the means of locomotor activity show the same negative correlation with alpha as known from single individuals. It is to be concluded that each individual has its characteristic place on the regression lines that describe the dependence of the three circadian parameters on the duration of wake time.