Institute of Medical Psychology, Faculty of Medicine, LMU Munich, Munich, Germany.
Groningen Institute for Evolutionary Life Sciences (GELIFES), University of Groningen, Groningen, The Netherlands.
J Pineal Res. 2021 Apr;70(3):e12723. doi: 10.1111/jpi.12723. Epub 2021 Mar 11.
In humans, the most important zeitgeber for entrainment is light. Laboratory studies have shown that meaningful changes in light exposure lead to phase shifts in markers of the circadian clock. In natural settings, light is a complex signal varying with external conditions and individual behaviors; nonetheless, phase of entrainment is assumed to be fairly stable. Here, we investigated the influence of season and weekly schedule (as indicators of variation in light landscapes) on phase of entrainment. Using a within-subjects design (N = 33), we assessed dim-light melatonin onset (DLMO) as a circadian phase marker in humans, on workdays and work-free days, in summer (under daylight saving time) and in winter, while also estimating sleep times from actimetry. Our mixed-model regressions show that both season and weekly structure are linked with changes in phase of entrainment and sleep. In summer, both DLMO and sleep times were about 1 hour earlier compared to winter, and sleep duration was shorter. On work-free days, DLMO and sleep times were later, and their phase relationship differed more relative to workdays. All these effects were stronger in later chronotypes (those who habitually sleep late). Our results confirm that phase of entrainment is earlier when stronger zeitgebers are present (summer) and show that it relates to midday or midnight rather than sunrise or sunset. Additionally, they suggest that late chronotypes are capable of rapid phase shifts each week as they move between workdays and work-free days, stimulating interesting questions about the stability of circadian phase under natural conditions.
在人类中，最重要的时间同步信号是光。实验室研究表明，光照暴露的显著变化会导致生物钟标记物的相位变化。在自然环境中，光作为一个复杂的信号随外部条件和个体行为而变化；尽管如此，同步相位被认为是相当稳定的。在这里，我们研究了季节和每周日程（作为光照景观变化的指标）对同步相位的影响。使用个体内设计（N=33），我们评估了暗光褪黑素起始（DLMO）作为人类的生物钟相位标记物，在夏令时的夏季和冬季的工作日和休息日，同时还通过活动记录仪估计睡眠时间。我们的混合模型回归表明，季节和每周结构都与同步相位和睡眠的变化有关。在夏季，与冬季相比，DLMO 和睡眠时间都提前了约 1 小时，睡眠时间也更短。在休息日，DLMO 和睡眠时间较晚，并且它们的相位关系相对于工作日差异更大。所有这些影响在较晚的生物钟类型（那些习惯晚睡的人）中更为强烈。我们的研究结果证实，当更强的时间同步信号存在时（夏季），同步相位会更早，并且它与中午或午夜有关，而不是日出或日落。此外，它们表明，当晚生物钟类型在工作日和休息日之间转换时，它们能够每周快速进行相位变化，这引发了关于自然条件下生物钟相位稳定性的有趣问题。
