The fulnesse of humors, corpus succiplenum, is the aliment or food of sleepe, as is to be seene in children and yong folkes; but the humors of old men are dried vp, as the stalkes of plants, and the corne in haruest, and their skinne rough, withered and wrinkled as old trees. Hence it is that they cannot sleepe soundly, but the crowing of the cocke, the noise of little birds, the whimpering of mice, euery small stirrage waketh them.
Source: wiktionary