Unnatural light-dark cycles expose duelling circadian clocks

In mammals, the endogenous daily pacemaker that regulates circadian rhythms like sleep and wakefulness is localized to a defined site in the brain, the suprachiasmatic nucleus (SCN), which is composed of many neurons whose circadian activities are in synchrony with one another. By exposing rats to a very short day/night schedule – a regimen that effectively pushes the limits of the SCN’s ability to set the clock to day length – researchers have discovered within the SCN two sub-clocks that normally oscillate in unison, but can become disconnected from one another as a result of artificial day/night cycles. One clock followed the artificially short 11-hr. day/11-hr. night schedule, while the other followed a longer cycle (>24 hrs.), but both clocks controlled behavioral rhythms within an individual animal.

The researchers, Horacio de la Iglesia and William Schwartz of the University of Massachusetts Medical School in Worcester, collaborating with Trinitat Cambras and Antoni Díez-Noguera of the University of Barcelona, found that the two locomotor activity rhythms reflected the separate oscillating activities of two areas within the SCN – essentially the top and bottom halves – that correspond to previously described anatomical subdivisions.

The results add to a growing awareness that it is a network of multiple oscillators, not only throughout the brain and body but also within the SCN itself, that underlies the workings of the circadian timing system. In humans, some of the symptoms arising from jet lag or rotating work schedules might not be due to the desynchronization between the central brain pacemaker and downstream oscillators in the body, but rather from the uncoupling of oscillators within the central pacemaker itself.

Horacio O. de la Iglesia, Trinitat Cambras, William J. Schwartz, and Antoni Díez-Noguera: “Forced Desynchronization of Dual Circadian Oscillators Within the Rat Suprachiasmatic Nucleus”

Publishing in Current Biology, Volume 14, Number 9 May 4, 2004.