It's probably because of jet lag.

可能是时差的原因。

youdao

I woke up 4 am because of jet lag and shops were open.

我因为时差原因在凌晨4 点钟醒了，发现商铺居然还开着门。

youdao

Well, I'm not a Buddhist, but when I came to China, I felt like I was a baby all over again. I arrived, and all I wanted to do was sleep because of jet lag.

虽然我不是佛教徒，但来中国之后，我感到自己仿佛又变成了一个彻头彻尾的婴儿——抵达中国后，因为时差反应，我一心只想睡觉；

youdao

And after flying all night, I am up way too early because of jet lag.

CNN: Behind the scenes: In the field, coffee a priority

Study researcher Renee Biss, a graduate student at the University of Toronto, said one of the reasons morning people may be happier is because they tend not to suffer from social jet lag.

CNN: STORY HIGHLIGHTS