Phase-Amplitude Resetting of the Human Circadian Pacemaker via Bright Light: A Further Analysis

Jan 1, 1994Journal of biological rhythms

Bright light can reset the timing of the human body clock: further analysis

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Abstract

A single 5-hour episode of bright light induces weak type 1 resetting of the human circadian pacemaker.

Two consecutive days of bright light exposure can lead to critical resetting, with significant amplitude reduction.
Three cycles of bright light induce strong type 0 resetting, affecting circadian amplitude based on the initial phase of application.
Stimulus centered near the low point of the temperature cycle results in large phase shifts through amplitude suppression.
When centered away from the temperature nadir, smaller phase shifts occur, accompanied by both slight increases and decreases in circadian amplitude.
The findings suggest that the human circadian pacemaker operates under a phase-amplitude model rather than a phase-only model.

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We present here an analysis of strong, weak, and critical bright-light resetting trials in humans, and report not only phase but also amplitude data for the first time. For this analysis, an appropriate iterative smoothing procedure for phase transition curves is introduced, in which the data are sequenced so as to minimize the perpendicular distance from the data to the smoothed fit. From these smoothed data, we create polar phase-amplitude resetting maps (PARMs) in order to fully illustrate the effects of the resetting stimuli on both circadian amplitude and phase, and thereby to determine whether these resetting results can be described by a phase-only model or whether a phase-amplitude model is required. Our results indicate that a single 5-hr episode of bright light induces weak type 1 resetting of the human circadian pacemaker. Two cycles of exposure to the same stimulus on consecutive days induce critical resetting, in which significant amplitude reduction may be observed. A three-cycle stimulus induces strong type 0 resetting with different effects on circadian amplitude, depending on the initial phase of the stimulus application. When a three-cycle stimulus is centered near the nadir of the temperature cycle, large phase shifts are achieved via amplitude suppression. However, when this stimulus is centered away from the temperature nadir, smaller phase shifts are achieved in which both small increases and small decreases in circadian amplitude are observed. These data indicate that the human circadian pacemaker is not a simple, phase-only oscillator. Instead, a full description of human circadian resetting responses to light requires analysis of both phase and amplitude data--a finding that is consistent with a phase-amplitude model of the circadian resetting mechanism.

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Phase-Amplitude Resetting of the Human Circadian Pacemaker via Bright Light: A Further Analysis

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