Production of the sleep hormone melatonin begins at dark.
睡眠荷尔蒙褪黑素在夜间开始分泌。
The animation uses a "hot metal" colour scale that begins at dark red and progresses through orange and yellow to white at the highest levels of activity.
影片里用色线标出了活动等级：由深红开始，逐步过渡桔红，再到黄色，进而显成白色这一最高等级。
Night frogs are extremely hard to find, coming out only at dark and during the monsoon season, living either in fast-flowing streams or on moist forest ground.
夜蛙往往极难发现，他们生活在水流湍急的溪流中或者潮湿的森林地带并且仅在雨季的夜间才会出来活动。
Night frogs are extremely hard to find, coming out only at dark and during the monsoon season, living either in fast-flowing streams or on moist forest ground.
夜蛙往往极难发现，他们生活在水流湍急的溪流中或者潮湿的森林地带并且仅在雨季的夜间才会出来活动。
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