Characteristics of insolation and photoperiod. (a) Daily insolation and photoperiod from 30 June 1999 to 31 July 2004 (+40.447°N -79.9517°W). (b) Cumulative photoperiod before intensive care unit (ICU) admission, according to season. (c) Degradation of light signal (lux) from outdoors to ICU bed on sunny and cloudy days. (d) Adjusted cumulative ICU insolation (expressed in kilowatt hours per square meter) according to season.