Sleep is also influenced by ambient light. In addition to sending visual information to the thalamus, the ganglion cells in the retina send signals to a the suprachiasmatic nucleus (SCN) of the hypothalamus, located below the thalamus. The suprachiasmatic nucleus is the body’s primary circadian “pacemaker” and contains cells that analyze the strength and duration of the light stimulus and send signals to the pineal gland when the ambient light level is low or its duration is short. In response, the pineal gland secretes melatonin, a powerful hormone that facilitates the onset of sleep.
睡眠还受到环境光线的影响。除了向丘脑传递视觉信息外，视网膜中的神经节细胞还会向下丘脑的视交叉上核（位于丘脑下方）发送信号。视交叉上核是人体主要的昼夜节律"起搏器"，其内部细胞能够分析光线刺激的强度与持续时间，当环境光线变暗或日照时间缩短时，就会向松果体发出信号。松果体随之分泌褪黑素——这种强效激素能有效促进睡眠发生。 thalamus：丘脑 ganglion cell：神经节细胞 hypo thalamus：下丘脑 Retina：视网膜 suprachiasmatic nucleus：视交叉上核 melatonin：褪黑素