賓夕法尼亞大學（University of Pennsylvania）行為睡眠醫學項目副主任菲利普?格曼認為，通常人們在夏天會感到更愉快。我們的晝夜節律（包括睡眠周期、荷爾蒙波動和體溫，大致以24小時為一個周期）主要受日光影響。格曼說：“人們在夏季的幾個月里會感覺更好，心理狀態有小小的提升，會有更多的正面情緒。”

但實際情況也不是千人一律。波士頓大學（Boston University）神經科學教授伊琳娜?V?日達諾娃博士說：“晝夜節律因人而異，所以日照的多少和時長對不同的人來說意義也不同。”比如，早晨的陽光會讓習慣晚醒的人感到不適。而對于習慣早起的人來說，早晨的陽光就會有積極的效果，傍晚的陽光倒是會讓他們不高興。

日達諾娃認為，這一點與飲食習慣類似。

她說： “有的人喜歡一日三餐，有的人喜歡一日十餐（每一餐的分量都較小）。在一天中的某個時段，對于有些人來說陽光是很好的興奮劑，但對另一些人來說，陽光不僅不能給人帶來正面情緒或者起到提神的效果，反而還會引起輕微的不快。”

更多的日照還能控制晝夜節律，在這個過程中，人體生物鐘可以根據體外的環境（比如自然的明暗周期）來進行自我調節。日達諾娃認為，由于白晝變長，人體晝夜節律通常會得到更好的控制，也就是說能夠更好地與自然光線的明暗變化保持一致，而后者影響著人們的睡眠質量與情緒。

但是格曼也表示，目前還不太清楚更為愉悅的情緒狀態到底是來自更佳的晝夜節律和睡眠質量，還是只跟日照有關。

布萊根婦女醫院（Brigham and Women’s Hospital）醫學生物鐘項目主任弗蘭克?謝爾認為，反過來看，冬天的日照較少確實會帶來一些問題。他說：“日照時間的延長或縮短確實會有顯著的影響。冬天，白晝變短，很有可能我們醒來時天還黑著。此時，晝夜節律系統還沒開始促使我們清醒并調整情緒。”

所以，在白晝最長的夏至日前后，盡情享受日光吧。白晝正在慢慢變短哦!（財富中文網）

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譯者：劉偉義

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.”

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.

It’s similar to meal habits, Zhdanova said.

“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.”

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.

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.

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.”

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