The daily, weekly, and seasonal cycles of body temperature analyzed at large scale

We performed large-scale analyses of circadian and infradian cycles of human body temperature, focusing on changes over the day, week, and year. Temperatures (<i>n</i>= 93,225) were collected using temporal artery thermometers from a Boston emergency department during 2009–2012 and were statistically analyzed using regression with cyclic splines. The overall mean body temperature was 36.7°C (98.1°F), with a 95% confidence interval of 36.7–36.7°C (98.1–98.1°F) and a standard deviation of 0.6°C (1.1°F). Over the day, mean body temperature followed a steady cycle, reaching its minimum at 6:00–8:00 and its maximum at 18:00–20:00. Across days of the week, this diurnal cycle was essentially unchanged, even though activities and sleeping hours change substantially during the weekly cycles of human behavior. Over the year, body temperatures were slightly colder in winter than summer (~0.2°C difference), consistent with most prior studies. We propose these seasonal differences might be due to ambient effects on body temperature that are not eliminated because they fall within the tolerance range of the thermoregulatory system. Over the year, bathyphase (daily time of minimum temperature) appeared to parallel sunrise times, as expected from sunrise’s zeitgeber role in circadian rhythms. However, orthophase (daily time of maximum temperature) and sunset times followed opposite seasonal patterns, with orthophase preceding nightfall in summer and following nightfall in winter. Throughout the year, bathyphase and orthophase remained separated by approximately 12 h, suggesting this interval might be conserved. Finally, although 37.0°C (98.6°F) is widely recognized as the mean or normal human body temperature, analysis showed mean temperature was &lt;37.0°C during all times of day, days of the week, and seasons of the year, supporting prior arguments that the 37.0°C standard has no scientific basis. Overall, this large study showed robust and consistent behavior of the human circadian cycle at the population level, providing a strong example of circadian homeostasis.

我们针对人体体温的昼夜节律（circadian）与次昼夜节律（infradian）周期开展了大规模分析，重点关注每日、每周以及年度的体温变化。本研究共纳入93225条体温数据，均采集于2009至2012年间波士顿某急诊科的颞动脉体温计，随后采用循环样条回归法进行统计分析。全体样本的平均体温为36.7℃（98.1℉），95%置信区间为36.7~36.7℃（98.1~98.1℉），标准差为0.6℃（1.1℉）。每日时段内，平均体温呈现稳定的周期性波动：最低值出现在6:00~8:00，最高值则出现在18:00~20:00。尽管人类行为的周周期中活动与睡眠时长存在显著差异，但整周内的昼夜节律周期并未出现明显变化。年度维度下，冬季体温较夏季略低，温差约为0.2℃，该结果与多数既往研究结论相符。我们推测，这类季节性体温差异可能源于环境对体温的影响：由于该影响处于体温调节系统的耐受范围内，因此未被完全抵消。年度维度下，体温谷相位（bathyphase，即每日体温最低的时刻）与日出时间呈现同步变化，这符合日出作为昼夜节律授时因子（zeitgeber）的已知作用。但体温峰相位（orthophase，即每日体温最高的时刻）与日落时间的季节变化模式则恰好相反：夏季时峰相位早于日落，冬季则晚于日落。全年范围内，谷相位与峰相位的间隔始终保持在约12小时，这提示该时间间隔可能具有保守性。尽管37.0℃（98.6℉）被广泛认为是人体平均正常体温，但本研究分析显示，每日、每周以及各季节的平均体温均低于37.0℃，这支持了既往观点：37.0℃这一标准体温并无科学依据。总体而言，这项大规模研究揭示了人群层面人类昼夜节律周期的稳定且一致的表现，为昼夜稳态（circadian homeostasis）提供了极具说服力的例证。