Why does the scent of lilies make me sleepy?

I am sat here, with the heady smell of lilies wafting all around me, and I am so incredibly sleepy... I can barely keep my eyes open!

(The lilies in question are in a bouquet that Josh's Dad brought round as an apology from Josh for biting Mum last week... wow, eight days ago now... time flies!)

Lilies are one of my favourite flowers... I adore their scent, and actually I have three pots of lily bulbs that all take their turn in sitting on the kitchen table during the summer... just so that I can be surrounded by their scent.

I just wonder why I always seems to get sleepy around them...