Production of the sleep hormone melatonin begins at dark.

睡眠荷尔蒙褪黑激素在天黑时开始产生。

Production of the sleep hormone melatonin (荷尔蒙褪黑激素) begins at dark.

睡眠荷尔蒙褪黑激素的生产在天黑时开始。

My blood ran cold when I heard a strange noise outside at dark night.

当我在漆黑的夜晚听到外面有奇怪的声音时，我的血液都凉了。

They turn the light on at dark.

他们在黄昏时开灯.