Everyone knows morning people and late-night owls. The variation in individual circadian rhythms is an anecdotal as well as experimentally verified fact. But, until now, to systematically study circadian differences (and thereby hope to rout out the underlying genetic causes), scientists have had to rely on prolonged behavioural observation.
To screen for and identify circadian rhythm variations in humans, the required period of lengthy observation is prohibitively costly and labor intensive.
Circumventing these technical limitations, Ueli Schibler and colleagues report in the open-access journal PLoS Biology a new method to measure circadian cycles in mammalian cells cultured from tissues other than the suprachiasmatic nucleus. The researchers took skin samples from humans and infected subsequently made tissue cultures with a virus engineered to report with a fluorescent signal when a certain host circadian rhythm gene was expressed. They found that their data jibed with the previously accepted length of the human circadian cycle: 24.5 hours. Because of the sensitivity of their method, Schibler and colleagues also confirmed that, for both humans and mice, circadian rhythms vary substantially between individuals. This suggests that the genetics of the circadian clock likewise varies between individuals.