Shakespeare, in his famous 18th sonnet, gushed over love saying: 'Shall I compare thee to a summer's day? Thou art more lovely and more temperate.'

Somewhat jaded by the time he penned sonnet number 148, he lamented: 'O cunning love! With tears thou keep'st me blind.'

Today's scientists will concede that love is indeed blind.

Had Shakespeare understood something about the science of love, he would have known that it was really the frontal cortex of his brain that was blinded while his stress hormones worked overtime.

Studies show that a large part of the frontal cortex located at the forehead - associated with judgment and planning - is deactivated when one is in love.

'The dysfunction of the frontal cortex could impair character judgment and blind you to the other person's flaws,' said DrFrancis Ngui, a senior consultant psychiatrist and medical director at Adam Road Medical Centre.

Indeed, love can more than blind the smitten. They become obsessive, losing sleep over it. They cannot eat and they cannot stop thinking about the other person.

Sorry, poet types, there is a rational explanation for all this.

Blame it partly on a drop in the brain chemical called serotonin, which keeps a person calm, said Professor Donatella Marazziti of Italy's Pisa University, in a 1999 study.

Serotonin, which controls mood, emotions, sleep and appetite, is produced in the central nervous system and intestinal tract.

Being in love is stressful too. Prof Marazziti found in later studies that both men and women in love have considerably higher levels of the stress hormone cortisol, which is produced by the adrenal glands located just above the kidneys.

And if cortisol is not enough to keep the blood pressure high, another stress hormone, norepinephrine, is released, which increases heartbeat and the feeling of excitement.

Testosterone levels vary in men and women who are in love too. Men were found to have lower levels of the male sex hormone testosterone, which makes them less aggressive, while women had higher levels of testosterone, which increases sexual drive.

However, the real stuff of love - the bonding, trust and attachment - comes from a chemical in the brain called oxytocin, otherwise known as the 'love hormone', said Dr Ngui.

This is induced by touching and hugging, and is produced mainly in the brain's hypothalamus. Oxytocin is then distributed to other parts of the brain and spinal cord.

Some scientists opine that romantic love is not an emotion, but a reward system.

American biological anthropologist Helen Fisher, an expert on the science of human attraction, found that when a person is in love, the ventral tegmental area (VTA) located in the middle of the brain - where the the reward system is located - becomes very active.

Her study was published in The Journal Of Neurophysiology in 2005.

The VTA is where the brain chemical called dopamine is released, inducing feelings of pleasure, want, craving and motivation when a person is in love.

Keeping love hormones active

As the novelty of new love wears out, these hormonal rages also start to simmer down and return to normal levels.

'It's like if you suffer a pain every day, over time it will no longer be that painful. It's the way the body adjusts to the other person as a stimulus,' said Associate Professor Chia Sing Joo, senior consultant and head of the department of general surgery at Tan Tock Seng Hospital.

This is why Prof Chia, who specialises in sexual dysfunction and subfertility, said couples need to have sex to keep the love hormones active and remind the body of the desire, bonding and attachment when they were first in love.

'Love alone is not enough. You must have some spice. If there's no love making, there's no kick,' he said.

Couples should also constantly create new activities together, like picking up a new sport or musical instrument, said Associate Professor Tsai Fen-Fang from the department of psychology at the National University of Singapore.

'The shared participation in novel activities helps create new excitement in the relationship,' Prof Tsai said.

When couples find themselves fighting all the time, they should put down their unsolved issues and do a 'time out' like going on a short trip to put them in a better mood.

'When in a good mood, couples can communicate better, increasing their chance of resolving issues,' she said.

This article was first published in Mind Your Body, The Straits Times.