Too much light before bedtime may be detrimental to sleep, but light, when given and removed at the appropriate times, has the potential to be a powerful non-pharmacological tool to advance the timing of sleep in adolescents and college students. Light therapy also has the potential to improve mood and depression in this population. Several studies have investigated the importance of controlling both morning and evening light exposures to achieve the desired outcomes. Data from Figueiro et al37 and Appleman et al38 suggest that a phase advance of approximately 2 hours can be obtained after 1 week of light treatment that includes morning short-wavelength light and evening orange-tinted glasses.
Appleman et al38 placed study participants on an advanced sleep-wake schedule; both groups were woken up 1.5 hours earlier than their typical wake times. One group received a lighting intervention designed to advance circadian phase: short-wavelength light in the morning and orange-tinted glasses that block optical radiation below 525 nm in the evening. The other group received a light intervention designed to delay circadian phase: orange-tinted glasses in the morning and short-wavelength light in the evening. The advancing light intervention group significantly advanced their circadian phase and the delaying light intervention group significantly delayed their circadian phase.
In a follow-up study, Figueiro et al37 placed 12 early/morning types and 11 late/evening types on a 2-week, advanced sleep-wake schedule, twice in a mixed design. During the intervention weeks, half of the participants were randomly assigned to receive an advancing light pattern: short-wavelength light in the morning and orange-tinted glasses in the evening. The other group received a delaying light pattern: short-wavelength light in the evening and orange-tinted glasses in the morning. After a 3-week washout period, they were placed on the opposite protocol. Daily 24-hour light-dark patterns were monitored with the Daysimeter.39, 40 DLMO was significantly delayed after the delaying light exposure pattern and significantly advanced after the advancing light exposure pattern for both early and late types, compared to the baseline week. The research team found that the light-dark pattern, not an individual’s chronotype, controls circadian phase. What was striking about these studies was how important it was to control total light exposures during waking hours, not just the light treatment intervention. Therefore, a lighting scheme designed to promote earlier sleep times in adolescents should also address the importance of controlling light outside the school environment.
A lighting intervention designed to deliver light starting mid-morning until the end of the school day, and provisions to reduce light exposures at home in the evening, may help shift the timing of sleep in adolescents and increase sleep duration. Successful light therapy in this population will likely involve both school administrators and family members.
To help lighting professionals select light sources and targeted photopic light levels that will increase the potential for circadian light exposure in schools and other buildings, the LRC developed a circadian stimulus (CS) calculator to determine CS for any combination of source type and light level in photopic lux. CS is a measure of the effectiveness of optical radiation on the retina for stimulating the human circadian system, representing the effectiveness of a given SPD to stimulate the human circadian system from threshold (CS = 0.0) to saturation (CS = 0.7). Our research shows that exposure to a CS of 0.3 or greater at the cornea, for at least one hour in the morning, is effective for stimulating the circadian system.