Ecological measurements of light exposure, activity and circadian disruption

In humans, circadian rhythms have an average period of 24.2 hours. The 24-hour patterns of light and dark on the retina synchronise circadian rhythms to the local time on earth. Lack of synchronisation between the endogenous clock and the local time has been associated with a host of maladies. Therefore, it is important to measure circadian light exposures over the course of the 24-hour day and to be able to assess circadian entrainment and disruption in actual living environments. Presented is an overview of the recently developed Daysimeter, a personal measurement device for recording activity and circadian light exposure. When the Daysimeter is worn on the head, two light sensors near the eye are used to estimate circadian light exposures over extended periods of time. Phasor analysis combines the measured periodic activity-rest patterns with the measured periodic light—dark patterns to assess behavioural circadian entrainment/disruption. As shown, day-shift and rotating-shift nurses exhibit remarkably different levels of behavioural circadian entrainment/disruption. These new ecological measurement and analysis techniques may provide important insights into the relationship between circadian disruption and well-being.