He prefers decaffeinated coffee in the evenings so it doesn't affect his sleep.

decaffeinated 是由前缀 de-（表示"去除"）和 caffeinated（含咖啡因的）组成的形容词。caffeinated 来自名词 caffeine（咖啡因）加上后缀 -ated（表示"含有...的"）。decaffeinated 字面意思是"去除了咖啡因的"，即"脱咖啡因的"。