NIOSH Training for Nurses on Shift Work and Long Work Hours

Effects of Light on Circadian Rhythms

• The light/dark cycle of the sun keeps our bodies synchronized with these predictable environmental changes. Light will even pass through closed eyelids during sleep and signal the circadian pacemaker.
• The circadian pacemaker is most sensitive to light in the morning and the evening, but note that light at these times has opposite effects. For persons on a regular schedule (sleeping at night), bright evening light causes a phase delay (getting sleepy later and waking up later). Bright morning light causes a phase advance (getting sleepy earlier in the evening and waking up earlier in the morning).
• The influence of light on the pacemaker is unpredictable about 2 to 4 hours before usual morning wake-up time; sometimes it causes phase delays and sometimes phase advances.
• Light during the middle of the day has less influence on the pacemaker, but exposure to bright light (such as sunlight) will increase the intensity of light needed to shift the pacemaker during the sensitive periods in the morning and evening and during the night. Also, getting some bright light during the middle of the day can improve daytime alertness as well as sleep at bedtime.
• In general, researchers estimate that light in the evening (about 2 hours before and after usual bedtime) can shift the circadian system about 2 hours later per day, whereas light in the morning (about 1 hour before and after usual wake-up time) can shift it about 1 hour earlier per day.