Rensselaer Polytechnic Institute
Light Measuring Device for Correcting Circadian Disruption
Mark S. Rea
The goal of this project has been to develop a personal, self-contained circadian light and activity measurement and feedback device to correct circadian sleep disorders through light prescriptions. The Daysimeter developed under this grant collects real-time data from calibrated light and activity sensors. These data are processed using phasor analysis to quantify the magnitude and angle of circadian entrainment exhibited by an individual.
Phasor analysis is a mathematical method to quantify the resonance between two periodic signals, and it was used under the present grant to measure the resonance between the light-dark and the activity-rest patterns obtained from the Daysimeter. A high degree of resonance between the light-dark pattern and the activity-rest pattern reflects a high degree of circadian entrainment as characterized by large phasor magnitudes.
The light-dark data are also used as input to a model of the light-sensitive human circadian clock. The model utilizes a van der Pol oscillator whereby the light input drives circadian phase output as measured by core body temperature, dim light melatonin onset (DLMO) or sleep times. The van der Pol oscillator model thereby provides a light prescription to users for either maintaining circadian entrainment or for shifting the circadian clock (advance or delay) to a new desired time.
To test acceptance by users of the Daysimeter technology as well as the relevant mathematical concepts, data were collected in collaboration with sleep researchers at Brown University from young adults with normally delayed sleep. Young adults were chosen for study because they commonly experience age-dependent delayed sleep phase with respect to socially acceptable activity-rest periods. All subjects were able to wear the Daysimeter for two weeks to provide baseline and post-intervention light and activity data without problems.
For the intervention, subjects were required to go to bed and to rise at fixed, earlier times during the second week. Using phasor analysis, the light and activity data were used to quantify circadian entrainment during the baseline and the post-intervention weeks. In addition to measuring the light and activity data, at the end of the baseline and post-intervention weeks subjects provided saliva samples for measurement of DLMO, an orthodox measure of circadian phase. Blood samples were also collected at the end of both weeks so that colleagues at Yale University could examine changes in gene expressionof nine circadian clock genes.
Circadian entrainment, as measured by phasor magnitude, improved slightly but not significantly from the baseline week to post-intervention week. Circadian phase, as measured by DLMO, advanced significantly. Expression of all circadian clock genes that were analyzed also changed significantly. Most importantly, it was shown that by measuring the actual light-dark exposure patterns during the baseline and post-intervention weeks, it was possible to predict the measured changes in circadian phase from the baseline week to the post-intervention week.
This study provides the first evidence that ecological light-dark exposure data can be used to predict circadian phase and as such can be the basis for personal light treatments to maintain circadian phase or to shift it to a different desired position. To help realize this vision, a prototype system was successfully demonstrated under this grant that wirelessly communicated data from the Daysimeter to a personal phone for processing. Moreover, the Daysimeter has been used successfully in additional populations, as reflected in the attached publication list. Technologies such as those developed under this grant may be used in the future to correct circadian sleep disorders, such as jet lag and shift work.