尼古拉斯·巴卡勒 楊綺帆

If you are a morning person， you may be at reduced risk for major depression， a new study suggests.

Several studies of the body’s circadian sleep-wake cycle have shown that being an early bird is associated with a lower risk for depression. But those studies were observational so could not prove cause and effect.

For example， people who are early birds may have other health or lifestyle behaviors that reduce their risk for depression—they may have a healthier diet， for example， exercise more， or have fewer health conditions， such as chronic pain， that are associated with depression. All these factors， and many others， could explain the decreased risk for depression， and not the fact of being an early bird. Moreover， depression itself causes sleep disturbances， so it could be that depression is a cause of being a night owl， rather than the other way around.

The new study， however， offers more compelling evidence that going to bed early and waking early may， in itself， provide protection against depression， independent of other factors. The study， published in JAMA Psychiatry， uses a research method called Mendelian randomization that helps pinpoint the cause of what may be a cause-and-effect relationship.

With Mendelian randomization， researchers can compare large groups of people based on genetic variants that are independent of other health or behavioral characteristics—in this case， the tendency to being a night owl or a morning person， inherited traits that are randomly allocated during our development in the womb. More than 340 genetic variants associated with circadian sleep rhythm have been identified， and the researchers can compare large groups of people with the genetic variants for being a morning person with groups that lack them. Nature has， in essence， set up the randomized experiment for them.

For the study， the scientists used two genetic databases of more than 800，000 adults to do a Mendelian randomization study of circadian rhythm and the risk for depression. They not only had genetic data， but also data on diagnoses of major depression and information on when people went to bed and woke up， collected with both self-reports and sleep laboratory records， which the researchers used to track the midpoint of sleep1， a helpful scientific measure of someone’s sleep tendencies. A morning person who tended to go to bed at 10 and wake up at 6， for example， would have a sleep midpoint of 2 a.m.

They found that in people with the genetic variants for being an early bird， for every hour earlier the sleep midpoint， there was a 23 percent lower risk of major depression.

Dr. Till Roenneberg， an expert in chronobiology who was not involved in the research， said a shortcoming of the study was that the scientists had no data on when these people had to rise for work or other obligations. Even with Mendelian randomization， he said， they can’t account for the fact that late types often need to go to work too early， which in itself may contribute to depression.

“They’ve drawn the right conclusions from their data，” he said， “but life is more complicated than that.”

If you are a night owl， will changing your habits alleviate depression or decrease the risk for developing it？ Not necessarily， said the lead author， Dr. Iyas Daghlas， a resident physician at the University of California， San Francisco. The study， he said， looks at large groups of people， not individuals.

“This data tells us that certain trends in society”—such as using smartphones and other blue light devices at night， which make us go to sleep later—“may be having an effect on the level of depression in the population，” he said. “These results do not say that if you go to sleep earlier， you’ll get rid of depression. Discovering which intervention in which populations will be effective—that has to be left to clinical trials.”

Still， he said， “While our data doesn’t tell us where the sweet spot2 is， I would say that if you’re an evening person， especially one who has to wake up early， advancing your bedtime about an hour or so is a safe intervention that might be helpful for your mental health.”

一項新的研究表明，如果你是一個習慣早起的人，那么你患重性抑郁癥的風險可能會降低。

關于人體生理醒睡周期的多項研究表明，早起的人患抑郁癥的風險更低。但這些研究是由觀察所得，因此無法證明存在因果關系。

例如，早起的人可能有其他影響健康的行為或生活方式可以降低罹患抑郁癥的風險——比如飲食更健康，鍛煉更頻繁，或很少有與抑郁癥相關的慢性疼痛等健康問題。這些因素和其他許多因素都可以解釋抑郁癥風險降低的原因，而并非早起這一個因素。此外，抑郁癥本身會導致睡眠障礙，因此抑郁癥可能是成為夜貓子的一個原因，而不是反過來。

然而，新的研究提供了更有說服力的證據(jù)，證明早睡早起本身可以預防抑郁癥，不受其他因素影響。這項研究采用了一種名為孟德爾隨機化的研究方法，有助于確定可能存在的因果關系中的起因。研究報告發(fā)表在《美國醫(yī)學會雜志·精神病學卷》上。

運用孟德爾隨機化方法，研究人員能夠基于獨立于其他健康或行為特征的遺傳變異來比對大量人群——本例指未來會變成夜貓子還是早起鳥——這些遺傳特征是胎兒在子宮內發(fā)育期間就被隨機賦予的。研究人員已經(jīng)確定了340多種與晝夜睡眠節(jié)律相關的遺傳變異，可以將攜帶“早起鳥”遺傳變異的大批人與缺乏這種遺傳變異的人進行比對。這本質上是大自然安排的隨機實驗。

在這項研究中，科學家們使用了兩個包含80多萬成年人的遺傳數(shù)據(jù)庫，對晝夜節(jié)律和抑郁癥風險進行了孟德爾隨機化研究。數(shù)據(jù)庫不僅含有基因數(shù)據(jù)，也有重度抑郁的診斷數(shù)據(jù)以及人們上床和醒來時間的信息。這些信息采集于自我報告和睡眠實驗室的記錄，研究人員用于追蹤“睡眠中點”——監(jiān)測某人睡眠傾向的一種有用的科學方法。例如，一個通常在晚上10點上床、早上6點醒來的人，“睡眠中點”是凌晨2點。

研究人員發(fā)現(xiàn)，攜帶早起鳥遺傳變異的人，“睡眠中點”每早一個小時，患重度抑郁的風險就降低23%。

時間生物學專家蒂爾·倫內貝格博士沒有參與這項研究，他說這項研究的一個缺點是，科學家們沒有這些人必須幾點起床工作或履行其他義務的數(shù)據(jù)。他說，即使采用孟德爾隨機化法，也無法解釋晚睡的人經(jīng)常需要過早上班的事實，而這本身可能會導致抑郁。

“他們從現(xiàn)有數(shù)據(jù)中得出了正確結論，”他說，“但生活遠比這復雜?！?/p>

如果你是一個夜貓子，改變習慣會減輕抑郁或降低患抑郁癥的風險嗎？該研究報告的主要作者、加州大學舊金山分校的住院醫(yī)生伊亞斯·達格拉斯博士的回答是，不一定。他說，這項研究針對的是大批人群，而非個人。

“現(xiàn)有數(shù)據(jù)告訴我們，社會的某些發(fā)展趨勢”——比如夜間使用智能手機和其他藍光設備，這會讓我們更晚入睡——“可能影響群體的抑郁程度。”他說，“這些研究結果并不是說早睡就能擺脫抑郁。要發(fā)現(xiàn)哪種干預措施對哪些群體有效，必須留給臨床試驗了。”

不過，他表示：“盡管現(xiàn)有數(shù)據(jù)沒有告訴我們最佳就寢時間，但我認為，如果你是一個晚睡的人，尤其是還得早起的話，一種有助心理健康的穩(wěn)妥干預辦法是，把就寢時間提前一小時左右。”

（譯者為“《英語世界》杯”翻譯大賽獲獎者）

1 midpoint of sleep 睡眠中點，即睡覺時間和起床時間之間的中間點。

2 sweet spot 最佳點，最有效點。