A three pulse phase response curve to three milligrams of melatonin in humans

Abstract

Exogenous melatonin is increasingly used for its phase shifting and soporific effects. We generated a three pulse phase response curve (PRC) to exogenous melatonin (3 mg) by administering it to free-running subjects. Young healthy subjects (n = 27) participated in two 5 day laboratory sessions, each preceded by at least a week of habitual, but fixed sleep. Each 5 day laboratory session started and ended with a phase assessment to measure the circadian rhythm of endogenous melatonin in dim light using 30 min saliva samples. In between were three days in an ultradian dim light (< 150 lux)-dark cycle (LD 2.5 : 1.5) during which each subject took one pill per day at the same clock time (3 mg melatonin or placebo, double blind, counterbalanced). Each individual's phase shift to exogenous melatonin was corrected by subtracting their phase shift to placebo (a free-run). The resulting PRC has a phase advance portion peaking about 5 h before the dim light melatonin onset, in the afternoon. The phase delay portion peaks about 11 h after the dim light melatonin onset, shortly after the usual time of morning awakening. A dead zone of minimal phase shifts occurred around the first half of habitual sleep. The fitted maximum advance and delay shifts were 1.8 h and 1.3 h, respectively. This new PRC will aid in determining the optimal time to administer exogenous melatonin to achieve desired phase shifts and demonstrates that using exogenous melatonin as a sleep aid at night has minimal phase shifting effects.

Figures

Figure 1. Diagram of a laboratory session

Subjects participated in two laboratory sessions. They slept on a fixed sleep schedule tailored to their habitual sleep times for 7 days before the first laboratory session and for 9 days in between the two laboratory sessions. During the phase assessments, on days 1 and 5, saliva was sampled every 30 min in dim light (

Figure 2. The three pulse phase response curve (PRC) to 3 mg of exogenous melatonin generated from subjects free-running during an ultradian LD cycle

Phase shifts of the DLMO are plotted against the time of administration of the melatonin pill relative to the baseline DLMO (top x-axis). The average baseline DLMO is represented by the upward arrow, the average baseline DLMOff by the downward arrow, and the average assigned baseline sleep times from before the laboratory sessions are enclosed by the vertical lines. Each dot represents the phase shift of an individual subject, calculated by subtracting the phase shift during the placebo session (free-run) from the phase shift during the melatonin session. The curved line illustrates the dual harmonic curve fit. The average clock time axis (bottom x-axis) corresponds to the average baseline sleep times. This PRC can be applied to people with different sleep schedules by moving the average clock time axis until the vertical lines align with the individual's sleep schedule.

Figure 3. Raw endogenous melatonin profiles from the individual subject who showed the largest phase delay to exogenous melatonin

The horizontal line represents the threshold used to calculate the DLMO and DLMOff using the crossings of the smoothed melatonin profiles (not shown here). The DLMO delayed by 3.1 h in the melatonin laboratory session and 0.4 h in the placebo laboratory session. Therefore, the net phase shift plotted in the PRC was a delay of 2.7 h.

Figure 4. Raw endogenous melatonin profiles from the individual subject who showed the largest phase advance to exogenous melatonin

The horizontal line represents the threshold used to calculate the DLMO and DLMOff using the crossings of the smoothed melatonin profiles (not shown here). The DLMO delayed by 0.2 h in the melatonin laboratory session and 2.8 h in the placebo laboratory session. Therefore, the net phase shift plotted in the PRC was an advance of 2.6 h.

Figure 5. Raw endogenous melatonin profiles from an individual subject who showed a minimal phase shift to exogenous melatonin

The horizontal line represents the threshold used to calculate the DLMO and DLMOff using the crossings of the smoothed melatonin profiles (not shown here). The DLMO delayed by 1.9 h in the melatonin laboratory session and 2.0 h in the placebo laboratory session. Therefore, the net phase shift plotted in the PRC was an advance of 0.1 h.