Search

Abstract Mathematical models of neurobehavioral function are useful both for understanding the underlying physiology and for predicting the effects of rest-activity-work schedules and interventions on neurobehavioral function. In a symposium [...]

Abstract A new focus for mathematical models of the circadian pacemaker involves the encapsulation within the models of detailed biological processes responsible for generating those circadian rhythms. Representing greater biological [...]

Zeitzer, Jamie M., Sat Bir S. Khalsa, Diane B. Boivin, Jeanne F. Duffy, Theresa L. Shanahan, Richard E. Kronauer, and Charles A. Czeisler. Temporal dynamics of late-night photic stimulation of the human circadian timing system. Am J Physiol Regul Integr Comp Physiol 289: R839-R844, 2005. First published May 12, 2005; doi:10.1152/ajpregu.00232.2005.--The light-dark cycle is the primary synchronizing factor that keeps the internal circadian pacemaker appropriately aligned with the environmental 24-h day. Although it is known that ocular light exposure can effectively shift the human circadian pacemaker and do so in an intensity-dependent manner, the curve that describes the relationship between light intensity and pacemaker response has not been fully characterized for light exposure in the late biological night. We exposed subjects to 3 consecutive days of 5 h of experimental light, centered 1.5 h after the timing of the fitted minimum of core body temperature, and show that such light can phase advance shift the human circadian pacemaker in an intensity-dependent manner, with a logistic model best describing the relationship between light intensity and phase shift. A similar sigmoidal relationship is also observed between light intensity and the suppression of plasma melatonin concentrations that occurs during the experimental light exposure. As with a simpler, 1-day light exposure during the early biological night, our data indicate that the human circadian pacemaker is highly sensitive even to typical room light intensities during the late biological night, with ~ 100 lux evoking half of the effects observed with light 10 times as bright. light; phase shift; melatonin suppression

It has been shown in animal studies that exposure to brief pulses of bright light can phase shift the circadian pacemaker and that the resetting action of light is most efficient during the first minutes of light exposure. In humans, multiple consecutive days of exposure to brief bright light pulses have been shown to phase shift the circadian pacemaker. The aim of the present study was to determine whether a single sequence of brief bright light pulses administered during the early biological night would phase delay the human circadian pacemaker. Twenty-one healthy young subjects underwent a 6.5-h light exposure session in one of three randomly assigned conditions: 1) continuous bright light of ~9,500 lux, 2) intermittent bright light (six 15-min bright light pulses of ~9,500 lux separated by 60 min of very dim light of melatonin; circadian pacemaker; photoreception; phase shift

The entrainment of totally blind individuals to a 24-hour day suggests that nonphotic stimuli can also synchronize the circadian rhythm. This possibility is investigated in 15 totally blind subjects by investigating the circadian rhythm of melatonin secretion and core body temperature changes in these individuals. Results reveal show that there is synchronization in nine out of the 15 subjects. This indicates that nonphotic stimuli also affect light-dark cycle entrainment.

A study was conducted to determine whether the phase-response curves had a 'dead zone' of insensitivity to bright light during the subjective day. The core body temperature measured one to two days after the application of the stimulus and the modulation of the phase-shifting effects of the bright-light stimulus were also investigated in the human samples studied. Human circadian pacemaker was found to be sensitive to light throughout subjective day even with the absence of transients.

Computer simulations of a quantitative model to study the effects of light on the human circadian system reveal the sensitivity of the system to light and the effect of light on the assessment of the intrinsic period of the circadian pacemaker. A proper assessment of the true intrinsic period of the circadian pacemaker necessitates low ambient light intensities during wake periods, a study over a period of at least 20 days, and desynchrony of the circadian oscillator and the light-dark cycle.

Working at night can cause disturbances in the sleep-wake cycle that lead to insomnia during the day and reduced alertness with fatigue while on the job at night. There are also medical consequences; people who rotate shifts over the long term have an elevated risk of developing heart disease, digestive problems, and in women, reproductive dysfunction. Even people who remain on a permanent night-shift for many years do not fully adjust. Researchers have discovered that the hypothalamus, a region of the brain, is responsible for regulating the body's circadian (24-hour) rhythms. For this reason, the hypothalamus has been called the pacemaker of the circadian rhythms. It was once thought that the hypothalamus 'set' the daily rhythms according to the timing of social interaction, but the authors have recently demonstrated that it responds instead to darkness and light, and can actually be 'reset' by up to 12 hours over a period of two to three days. In this study, eight young men were exposed to bright light at night and almost total darkness during the day in an attempt to improve their adjustment to night work. Circadian adaptation was measured by monitoring body temperature, which is normally at its lowest in the middle of the night; if the subjects were to adapt to night work it would shift to reach a 24-hour low during the day. This adaptation was in fact achieved. The low temperature occurred in the midafternoon after four days of the treatment, which consisted of exposure to bright lighting at night and darkness while sleeping during the day. Other physiologic signs confirmed the conclusions based on body temperature. The subjects also reported improved alertness and mental functioning at night. Therefore, manipulation of lighting conditions can correct the maladaptation of the human circadian rhythms to night work. (Consumer Summary produced by Reliance Medical Information, Inc.)

Rimmer, David W., Diane B. Boivin, Theresa L. Shanahan, Richard E. Kronauer, Jeanne F. Duffy, and Charles A. Czeisler. Dynamic resetting of the human circadian pacemaker by intermittent bright light. Am J Physiol Regulatory Integrative Comp Physiol 279: R1574-R1579, 2000.--In humans, experimental studies of circadian resetting typically have been limited to lengthy episodes of exposure to continuous bright light. To evaluate the time course of the human endogenous circadian pacemaker's resetting response to brief episodes of intermittent bright light, we studied 16 subjects assigned to one of two intermittent lighting conditions in which the subjects were presented with intermittent episodes of bright-light exposure at 25- or 90-min intervals. The effective duration of bright-light exposure was 31% or 63% compared with a continuous 5-h bright-light stimulus. Exposure to intermittent bright light elicited almost as great a resetting response compared with 5 h of continuous bright light. We conclude that exposure to intermittent bright light produces robust phase shifts of the endogenous circadian pacemaker. Furthermore, these results demonstrate that humans, like other species, exhibit an enhanced sensitivity to the initial minutes of bright-light exposure. circadian rhythms; core body temperature; phototherapy

Bright Ligh Induction of Strong (Type 0) Resetting of the Human Circadian Pacemaker THIRTY YEARS AGO, A PHASE RESPONSE curve (PRC) to light was first described [1], revealing the mechanism [...]