After a long drive, he started feeling sleepiness and had to pull over to rest.

sleepiness 是名词，由形容词 sleepy（困倦的）加上名词后缀 -ness 构成，表示"困倦的状态"。sleepy 本身由 sleep（睡觉）加形容词后缀 -y 构成，表示"想睡觉的"。