Valentine's Day: 5 things you didn't know about love and sex

And to mark 14 February, here are five things you might not know about love and sex.

Aphrodisiac food
Chocolate contains phenylethylamine, a chemical that causes the release of dopamine and norepinephrine, the key hormones that help us fall in love.

Instant attraction
It only takes between 90 seconds and four minutes to decide if you fancy someone, and 55 per cent of attraction is based on body language alone.

Monogamous animals
Only three per cent of mammals mate for life. Animals that do find a lifelong partner include gibbons, swans, wolves, albatrosses, penguins, eagles and termites.

Natural painkiller
Simply looking at a photo of the one you love, or even thinking about them, increases levels of oxytocin, the body's natural painkiller.

Broken Heart Syndrome
Also called stress-induced cardiomyopathy, it is caused by an emotionally stressful event, such as a break-up, and causes sudden intense chest pain.