Waking up is perfectly simple if you are a naturally early riser. But for many, the alarm clock is a sworn enemy and over-sleeping is a regular problem.

But now there is a perfect excuse for those people staying in bed.

Researchers at Leiden University in the Netherlands have found that the internal biological clock which makes us sleepy at night and wakeful during the day can be set to different times.

Late to the office again?

People who are barely awake before midday but full of life during the evening could have their clocks running as much as two hours behind the early risers.

By measuring changes in body temperature the researchers found that the clock can be set at different times relative to sunset.

It means some people's bodies simply wake up later than others and there is little that can be done to alter their natural disposition.

Even if they are forced to start work early, they always sleep in late whenever they can.

The bad news is that the excuse only works for the few people whose clocks are seriously behind time. Most people are set to operate in the middle of the day and should not have a problem getting up any time after sunrise.