Purple-blue flowers from Greece

by Edwina
(Kefalonia Greece)

unknown

This plant is growing in Kefalonia Greece,
it has five petals per head which close at night and the leaves are long & narrow, being light green on top with red veins underneath.
The stem is dark red and at it's lowest point resembles bamboo. It is at present in full sun till late afternoon and in a pot with compost. We would like to know the name of it so we can care for it properly. Thank you in advance for any help you can offer.