This is a natural cycle of life.

这是自然的生命周期。

David: Some think it's part of the earth's natural cycle.

大卫：有些认为是地球自然循环的一部份。

All of this is a natural cycle that everyone must go through.

但所有这些不过是每个人都要经历的自然周期罢了。

This will help combat insomnia by giving in to your body's natural cycle.

适应你身体自然循环后，这将有助于抗击失眠。