Lettuce has a long tradition of sending people to sleep. It used to have the reputation too of making girls drop their knickers guard and leading to unintended pregnancies. I thought there was a bit in one of shakespeare's plays where a maid falls pregnant, and someone comments, "' . . .and she only went down the garden to cut a lettuce", which is greeted with laughter by other characters, but I can't find it.
I did find this from QI, though:-
"- The thing that is long, begins with "L", and gets you horny, sleepy and pregnant is lettuce. Hippocrates, the father of medicine, described the opiate properties of lettuce, as did Beatrix Potter in Peter Rabbit. Lettuce is soporific, and Peter nearly ends up in Mrs. McGregor's rabbit pie because he falls asleep after eating too much lettuce. Lettuce has seen been bred to be less soporific. Wild lettuce however contains tropane alkaloid, which is also in cocaine. Americans did try to sell this lettuce under the names of "L'Opium" and "Lettucene", but most of it was made out of ordinary garden lettuce. Lettuce should not be fed to rabbits because it upsets their stomachs. Victorians picnickers wrapped lettuce around butter to keep the butter fresh."