Lime trees are easy to spot on a sunny day, they appear to be almost glowing. They have heart shaped leaves.
Take one handful of fresh flowers and bash about a bit with mortar and pestle. Put the bashed flowers in a teapot and fill with one mug boiling water. Leave to cool and do its stuff overnight.
Drink.
If you dry the limeflowers and mix with dried lavender and use in your pillow it also can help those who have problems dropping off.

Limeflowers.