In general, people are more cheerful in the summertime, according to Philip Gehrman, associate director of the Behavioral Sleep Medicine Program at the University of Pennsylvania. Light serves as the strongest cue to regulate circadian rhythms, which include the sleep cycle, hormonal fluctuations and body temperature and follow a roughly 24-hour pattern. “People tend to feel better in summer months,” Gehrman said. “There’s a slight elevation in our mood. More positive emotions are reported.”

宾夕法尼亚大学（University of Pennsylvania）行为睡眠医学项目副主任菲利普•格曼认为，通常人们在夏天会感到更愉快。我们的昼夜节律（包括睡眠周期、荷尔蒙波动和体温，大致以24小时为一个周期）主要受日光影响。格曼说：“人们在夏季的几个月里会感觉更好，心理状态有小小的提升，会有更多的正面情绪。”

But the effect isn’t uniform—circadian rhythms vary from person to person, so the amount and timing of sunlight means different things to different people, said Dr. Irina V. Zhdanova, a neuroscience professor at Boston University. For example, morning sunlight can irritate people who wake up late. On the other hand, for people who rise early, morning sunlight can have a positive effect, but sunlight may irritate them in the evening.

但实际情况也不是千人一律。波士顿大学（Boston University）神经科学教授伊琳娜•V•日达诺娃博士说：“昼夜节律因人而异，所以日照的多少和时长对不同的人来说意义也不同。”比如，早晨的阳光会让习惯晚醒的人感到不适。而对于习惯早起的人来说，早晨的阳光就会有积极的效果，傍晚的阳光倒是会让他们不高兴。

It’s similar to meal habits, Zhdanova said.

日达诺娃认为，这一点与饮食习惯类似。

“Some like three meals a day, others like 10 small meals a day,” Zhdanova said. “Sometimes sunlight is good and uplifting during a certain time of the day, while for others, it’s neither uplifting or positive and can induce mild irritation.”

她说： “有的人喜欢一日三餐，有的人喜欢一日十餐（每一餐的分量都较小）。在一天中的某个时段，对于有些人来说阳光是很好的兴奋剂，但对另一些人来说，阳光不仅不能给人带来正面情绪或者起到提神的效果，反而还会引起轻微的不快。”

The extra sunlight also entrains the circadian rhythm, a process in which the internal biological clock aligns itself to external cues, like the light-dark cycle. Now that the days are longer, in general, the circadian rhythm is entrained much better, Zhdanova said. In other words, the circadian rhythm is better aligned with natural sunlight and darkness, which can affect people’s sleep and moods.

更多的日照还能控制昼夜节律，在这个过程中，人体生物钟可以根据体外的环境（比如自然的明暗周期）来进行自我调节。日达诺娃认为，由于白昼变长，人体昼夜节律通常会得到更好的控制，也就是说能够更好地与自然光线的明暗变化保持一致，而后者影响着人们的睡眠质量与情绪。

However, Gehrman said, it is unclear whether elevated moods come from more entrained circadian rhythms and better sleep, or if the happiness boost is directly attributable to the sunlight.

但是格曼也表示，目前还不太清楚更为愉悦的情绪状态到底是来自更佳的昼夜节律和睡眠质量，还是只跟日照有关。

On the flip side, experiencing less sunlight in the winter can pose a challenge, said Frank Scheer, director of the Medical Chronobiology Program at Brigham and Women’s Hospital. “The lengthening or shortening of the light period during the day does have a clear effect,” Scheer said. “In wintertime, when the days are shorter, we are more likely to wake up when it’s still dark outside and before the circadian system stimulates wakefulness and improved mood.”

布莱根妇女医院（Brigham and Women’s Hospital）医学生物钟项目主任弗兰克•谢尔认为，反过来看，冬天的日照较少确实会带来一些问题。他说：“日照时间的延长或缩短确实会有显著的影响。冬天，白昼变短，很有可能我们醒来时天还黑着。此时，昼夜节律系统还没开始促使我们清醒并调整情绪。”

So, enjoy the longest day of the year while it lasts—the days will start getting shorter again from here.

所以，在白昼最长的夏至日前后，尽情享受日光吧。白昼正在慢慢变短哦!