Chamomile flowers, lemon balm, lavender and catnip are the backbone of almost every bedtime blend ever sold, and all four grow happily on a Farmstand. Lettuce belongs here too, which surprises people: the bitter sap in a mature stem gave it a sleep reputation across several unrelated traditions long before anyone put it in a salad.
The reason to grow these rather than buy them is that flowers and soft leaves lose their aromatics fastest. A chamomile flower picked this morning smells of apples; a teabag smells of hay. Pick a small handful after dinner, steep five minutes, and you have something you cannot buy.