Daytime melatonin and light independently affect human alertness and body temperature

Renske Lok,Marijke C M Gordijn,Minke J Van Koningsveld,Roelof A Hut,Domien G M Beersma

doi:10.1111/jpi.12583

Abstract

Light significantly improves alertness during the night (Cajochen, Sleep Med Rev, 11, 2007 and 453; Ruger et al., AJP Regul Integr Comp Physiol, 290, 2005 and R1413), but results are less conclusive at daytime (Lok et al., J Biol Rhythms, 33, 2018 and 589). Melatonin and core body temperature levels at those times of day may contribute to differences in alerting effects of light. In this experiment, the combined effect of daytime exogenous melatonin administration and light intensity on alertness, body temperature, and skin temperature was studied. The goal was to assess whether (a) alerting effects of light are melatonin dependent, (b) soporific effects of melatonin are mediated via the thermoregulatory system, and (c) light can improve alertness after melatonin‐induced sleepiness during daytime. 10 subjects (5 females, 5 males) received melatonin (5 mg) in dim (10 lux) and, on a separate occasion, in bright polychromatic white light (2000 lux). In addition, they received placebo both under dim and bright light conditions. Subjects participated in all four conditions in a balanced order, yielding a balanced within‐subject design, lasting from noon to 04:00 pm. Alertness and performance were assessed half hourly, while body temperature and skin temperature were measured continuously. Saliva samples to detect melatonin concentrations were collected half hourly. Melatonin administration increased melatonin concentrations in all subjects. Subjective sleepiness and distal skin temperature increased after melatonin ingestion. Bright light exposure after melatonin administration did not change subjective alertness scores, but body temperature and proximal skin temperature increased, while distal skin temperature decreased. Light exposure did not significantly affect these parameters in the placebo condition. These results indicate that (a) exogenous melatonin administration during daytime increases subjective sleepiness, confirming a role for melatonin in sleepiness regulation, (b) bright light exposure after melatonin ingestion significantly affected thermoregulatory parameters without altering subjective sleepiness, therefore temperature changes seem nonessential for melatonin‐induced sleepiness, (c) subjective sleepiness was increased by melatonin ingestion, but bright light administration was not able to improve melatonin‐induced sleepiness feelings nor performance. Other (physiological) factors may therefore contribute to differences in alerting effects of light during daytime and nighttime.

Highlights

The suprachiasmatic nucleus (SCN) is the pacemaker of the mammalian circadian timing system
Differences in melatonin and core body temperature levels at those times of day might contribute to different responses in alertness
The goal of this experiment was to assess whether (a) alerting effects of light are melatonin suppression dependent, (b) these effects were mediated via the thermoregulatory system, and (c) light‐induced alertness depended on sleepiness levels during daytime

Summary

Introduction

The suprachiasmatic nucleus (SCN) is the pacemaker of the mammalian circadian timing system. Many physiological rhythms are regulated by the SCN, like melatonin secretion and core body temperature.[1,2] Plasma melatonin rises during the evening and peaks at night, while levels are virtually zero during daytime in most people.[3] Core body temperature (CBT) peaks in the evening and has its nadir in the early morning.[4] Correlations between patterns of melatonin and CBT have been shown.[5,6] Alertness, which is associated with high levels of environmental awareness, fluctuates in a circadian manner,[7] with lowest levels in the early morning, and high levels during daytime. Light‐induced melatonin suppression is associated with decreased sleepiness,[8,9] probably not at intermediate indoor light levels.[10,11] Melatonin ingestion increases subjective sleepiness and lowers CBT,[12] but CBT manipulations affect subjective sleepiness at times of day when melatonin is virtually absent, suggesting a relationship between CBT and sleepiness independent of melatonin.[13]

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Results

Conclusion