I have found out what lemon balm is good for. It is so useful I immediately cut a bunch of it to dry- because the insects are already attacking (I don't have much whitefly this year, nor cabbage moths- because I didn't plant any brassicas- but there are little leaf hoppers, mealy bugs and a few aphids) and it was beginning to look poorly. I forestalled their ruining the whole plant by harvesting it (also cut and hung to dry a ton of sage, dill and thyme are next).
I decided it was worth a try, and made a simple tea. Steeped a few leaves in a small mug with a bit of honey. It definitely has an effect on me. Makes me quite drowsy and when I lie down, my arms and legs feel pleasantly heavy, not restless. I have only woken in the middle of the night a few times this week of trying lemon balm tea, and when I do I go straight back to sleep no problem. I feel so much more rested and alert when I wake in the morning.
In fact, I've quit drinking coffee (for other reasons too). I find that if I sleep soundly, I don't need a stimulant in the morning. A cup of chai or choc mint tea is fine. A few times my kid has complained about being unable to get to sleep (well past ten pm) I gave her just a sip or two of my tea and she goes to sleep very quickly after that. She doesn't like the lemon-honey taste but just a sip is sufficient. For myself, only two mature leaves, or one sprig of small ones, is enough for the calming effect. Lemon balm is reportedly harmless, as innocuous as chamomile (which I personally don't care for). I'm a new fan.
I should have guessed it's in the mint family, the leaves are very similar to mints and the stem has that square-sided shape. I have another mint now- spearmint or peppermint that I dug up from a friend's garden to replace mine that died this past winter. It's been in the water jar long enough roots are growing out- but I ran out of potting soil. Will pot it up as soon as I can.