Effect of Light on Human Circadian Physiology

Abstract

The circadian system in animals and humans, being near but not exactly 24-hours in cycle length, must be reset on a daily basis in order to remain in synchrony with external environmental time. This process of entrainment is achieved in most mammals through regular exposure to light and darkness. In this chapter, we review the results of studies conducted in our laboratory and others over the past 25 years in which the effects of light on the human circadian timing system were investigated. These studies have revealed, how the timing, intensity, duration, and wavelength of light affect the human biological clock. Our most recent studies also demonstrate that there is much yet to learn about the effects of light on the human circadian timing system.

Figure 1

Phase response curve to a single 6.5-hour episode of bright light in young adults. Phase shifts (in hours) of the plasma melatonin rhythm are plotted with respect to the circadian phase at which the center of the 6.5-hour light stimulus was presented. By convention, phase delay shifts (shifts to a later hour) are plotted as negative numbers, while phase advance shifts (shifts to an earlier hour) are plotted as positive numbers. Phase shift magnitude was determined by assessing phase before and after the light stimulus. Circadian phase of the light stimulus was defined relative to the midpoint of the plasma melatonin rhythm (defined as 22h) assessed just prior to the stimulus. Data from circadian phase 6–18 are double-plotted for better visualization. The open circle represents a subject whose phase shift was determined using salivary melatonin. The solid line represents a best-fit dual harmonic function to the data points. The dashed horizontal line represents the assumed 0.54-hour average phase delay drift of the human circadian pacemaker between the pre- and post-stimulus phase assessments. Figure reproduced with permission from Reference 42 [Khalsa SBS, Jewett ME, Cajochen C, and Czeisler CA. A phase response curve to single bright light pulses in human subjects. J Physiol (Lond) 2003; 549(Pt 3):945–952].

Figure 2

Phase shifts (panel A) and melatonin suppression (Panel B) in response to a 3-cycle 5-hour light stimulus in young adults. The magnitude of the phase shift (in hours) is plotted with respect to the illuminance of the light stimulus (in lux). Symbols represent mean (± s.e.m.) responses from each group of 7–9 subjects. The solid line represents a 3-paramater logistic curve fit to the data, and the upper and lower 95% confidence intervals of this fit are shown in the dashed lines. Figure reproduced with permission from Reference 63 [Zeitzer JM, Khalsa SB, Boivin DB, Duffy JF, Shanahan TL, Kronauer RE, and Czeisler CA. Temporal dynamics of late-night photic stimulation of the human circadian timing system. Am J Physiol 2005; 289(3):R839–R844].

Figure 3

Phase resetting responses to a 3-cycle 5-hour stimulus in young adults. The magnitude of the phase shift (in hours) is plotted with respect to the relative duration of the bright light exposure. The 0% group was exposed to complete darkness throughout the 5-hour stimulus and the 100% group was exposed to continuous bright light throughout the stimulus. Two groups received intermittent bright light exposure during the 5-hour stimulus: 5.3 minutes of bright light interspersed with 19.7 minutes of darkness (31% relative duration group); or 46 minutes of bright light interspersed with 44 minutes of darkness (63% relative duration group). Each point represents the response from 1 subject, and the horizontal bars represent the median phase shift for the group. Figure reproduced with permission from Reference 70 [Rimmer DW, Boivin DB, Shanahan TL, Kronauer RE, Duffy JF, and Czeisler CA. Dynamic resetting of the human circadian pacemaker by intermittent bright light. Am J Physiol 2000; 279(5):R1574–R1579]