Sentence:  It is said that the effect of eating too much lettuce is “<WSD>soporific</WSD>.” "I" have never felt sleepy after eating lettuces; but then "I" am not a rabbit. They certainly had a very <WSD>soporific</WSD> effect upon the Flopsy Bunnies! 
Meaning of the word marked with <WSD> tags:  Tending to induce sleep.