In terrestrial mammals, circadian rhythms are regulated by the interaction of the internal biological clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus with the earth’s natural 24-hour light-dark pattern (Refinetti 2006
). The SCN are self-sustaining oscillators with an intrinsic period that is typically slightly longer or shorter than 24 hours. The timing of the SCN is set by the local light-dark pattern, usually ensuring that the organism’s behavioral and physiological rhythms are synchronized with its photic niche (nocturnal, diurnal, or crepuscular).
Light incident on human retinas will entrain or phase shift SCN timing, depending upon the time, duration, spectrum and intensity of the stimulus (Stevens & Rea 2001
). These fundamental light characteristics affect the circadian system differently than they affect the visual system. Although we now know the human circadian system is more sensitive to light than was originally thought (Lewy et al. 1980
), it is much less sensitive to light than the visual system (Rea et al. 2002
). It is also well established that the human circadian system is maximally sensitive to short-wavelength (450 nm to 480 nm) light (Brainard et al. 2001
, Thapan et al. 2001
, and Rea et al. 2005
). Most electric light sources illuminating our indoor environments are designed to support the visual system by providing relatively low levels of light dominated by wavelengths near 555 nm, the peak of the photopic luminous efficiency function (CIE 1978
). Moreover, for convenience, electric light sources are available night or day and for variable durations. More and more then, people throughout the world are living under a roof illuminated by electric light sources, exposing them to dim days and extended dim light at night, separating them from the robust, natural light-dark cycle.
Studies have shown that adolescents report going to bed later as they get older (Crowley et al. 2007
). These age-related changes in bedtimes have been associated with reduced parental influence on bedtimes, increased homework and extra-curricular activities, and other activities such as playing on computers and watching television. Evidence to date supports the hypothesis that adolescents have a late circadian phase, contributing to these late bed times. With a highly structured school schedule requiring early rising, these adolescents typically experience reduced sleep durations. Indeed, on unrestricted weekends, adolescents rise 1.5 to 3 hours later than they do on weekdays (Crowley et al. 2007
Light during the day is important for entrainment; that is, for aligning circadian phase to the rest-activity cycle required by attending school. For reasons described above, however, electric lighting, including that common in schools, may not provide adequate light for circadian entrainment. Without a robust light stimulus during the day then, adolescents would logically be expected to exhibit late circadian phase and therefore go to bed late and experience restricted sleep.
Daily morning short-wavelength light exposures (after minimum core body temperature) are expected to slightly advance the clock every day and thereby maintain entrainment to the solar day (Jewett et al. 1997
). The impact of reduced daily short-wavelength light exposure on the circadian system of young adults, as might be experienced by students without adequate daylight (or electric light) exposure, has never been formally investigated. A simple before-and-after, within-subjects field experiment was conducted in a school with documented good daylight design to determine whether removal of short-wavelength light on five consecutive school days would delay circadian phase relative to a baseline measurement obtained prior to the intervention.