A quarter of the world's population is subjected to a 1 hr time change twice a year (daylight saving time, DST). This reflects a change in social clocks, not environmental ones (e.g., dawn). The impact of DST is poorly understood. Circadian clocks use daylight to synchronize (entrain) to the organism's environment. Entrainment is so exact that humans adjust to the east-west progression of dawn within a given time zone [1]. In a large survey (n = 55,000), we show that the timing of sleep on free days follows the seasonal progression of dawn under standard time, but not under DST. In a second study, we analyzed the timing of sleep and activity for 8 weeks around each DST transition in 50 subjects who were chronotyped (analyzed for their individual phase of entrainment [2]). Both parameters readily adjust to the release from DST in autumn but the timing of activity does not adjust to the DST imposition in spring, especially in late chronotypes. Our data indicate that the human circadian system does not adjust to DST and that its seasonal adaptation to the changing photoperiods is disrupted by the introduction of summer time. This disruption may extend to other aspects of seasonal biology in humans.
BROWN, F. A. (1959). The rhythmic nature of animals and plants. American Scientist, 147-168.
Brown, F. A. (1972). The "clocks" timing biological rhythms. American Scientist, Vol. 60, No. 6, 756-766.
Recent discoveries suggest that the mysterious biological clock
phenomenon results from a continuous interaction between organisms and
the subtle geophysical environment. Presents an alternative hypothesis to that of the fully independent,
internal "biological clock" to explain the rhythmic activities of living
things. It is noted that extensive investigation has failed to identify
such a clock. In addition, newly disclosed sensitivities of organisms
to the subtle geophysical environment and advances in knowledge of
properties of clock-timed rhythms suggest the need for critical
re-examination of the original concept. It is proposed that the observed
phenomena result from a continuous interaction between organisms and
the subtle geophysical environment in which time is "read" from the
environment and employed to time adaptive, recurring physiological
patterns.
