We've all heard them -- right from when we were kids and mum scolded us for reading in bad light to the beauty on TV who swears by guzzling litres upon litres of water to purify and cleanse -- health mantras that have been handed down over the ages. But do they hold any value health-wise?

To answer just that, we took a look at some of the most common rules we live by and find out if they're true or just urban legends.

~ Eight glasses of water a day 
They all recommend it -- the catwalk model with the perfect figure, the actress with the flawless complexion, the fitness freak with abs of steel and everyone in between. And most people buy into it too. Every second teenager is guzzling water in the hope of washing out their system, detoxifying the body, and what have you. But does it actually work?

According to health portal Mayoclinic.com, while there are recommended levels of water intake, it is by no means a one-size-fits-all solution. Factors like diet, body type, environment and weather among others have quite a say in how much water you should ideally drink.

If your diet is high in other sources of water -- beverages (soups and juices, even tea and coffee qualify here), fruit and vegetables (watermelon, cucumbers are very high in water content) you can comfortably knock off a glass or two.

If you lead a sedentary lifestyle -- long hours at the computer and little to no exercise, your requirement goes down a bit too.

So, drink when you're thirsty and not just to complete your day's quota.

~ If you're cold, you'll catch a cold
The reason one catches the common cold is due to exposure to the cold virus -- contact with a person already suffering from a cold or contact with a contaminated surface. Cold temperatures will not affect your immunity in any way, unless in extreme cases or hypothermia.

• 7 easy ways to avoid a cold

If you do want steer clear of the heavy head and clogged nasal passages that usually accompany a cold, you might want to read this!

~ A bad day will make you grey
There's this joke doing the rounds online: A mother says to her young daughter, "See this strand of grey hair? Whenever you're naughty and have me worried, another strand turns grey." To which the kid promptly replies, "You must have troubled Grandma a lot when you were little, she has no black hair left at all!"

Which one of us hasn't heard that stress turns hair grey? Chances are, almost all of us have. What might come as a surprise though is that while almost everyone subscribes to that belief, there is no actual scientific evidence to support it.

• Beat that headache with yoga!

Greying is just another sign of growing old, like wrinkles. It's in our genes. While some of us might go grey in our 60s, others might show hints of graying as early as their late teens. So while getting strung out about something might not actually turn your youthful locks to silver, it's just as good an excuse to keep your cool.

~ Reading in poor light will ruin your eyes
Another old wives' tale this. Yes, reading in poor light can tire the eyes, says palmereye.com, but when it comes to actually damaging your eyesight, poor light cannot be named as a factor. This hold true for television viewing and long hours staring at the computer screen too. Prolonged periods will fatigue the eyes, and might cause slight watering but long-term damage is usually very rare.

~ Coffee is bad for you
True, coffee is a diuretic (it tends to increase the flow of urine, which causes the body to get rid of water). However, according to a report on msnbc.com, your intake of water through the day will more than make up for the water lost due to a cup or two of coffee.

The caffeine content in coffee is also said to be addictive. While regular coffee drinkers do complain of headaches and drowsiness if they suddenly give up their daily brew, a gradual decrease in intake over a few days can just as easily keep those headaches at bay.

However, when it comes to pregnant women or people suffering from stress-related diseases, consulting a doctor would be best.