Are you a night owl? This may be the reason why...

It's a well known fact that some people are early birds and some prefer to stay up late and sleep in.

But now a new study has found that a genetic mutation might be to blame for keeping you awake at night.

It's called the ‘CRY1′ gene, known to play a role in the circadian clock.

The circadian clock is an internal biological clock that normally dictates when you feel sleepy each night and when you're ready to wake up.

Scientists say that people with variants of the gene have a longer sleep-wake cycle, which keeps you up longer.

People who identify as night owls are often diagnosed with delayed sleep phase disorder (DSPD), meaning their 24-hour sleep-wake cycle is delayed.

Most people with DPSD are forced to wake up before their bodies tell them to in order to make it to work or school on time, which can lead not only to insomnia early in the night but also to fatigue during the day.

The study, conducted at Rockefeller University in New York City, asked subjects with DPSD to spend two weeks in a laboratory apartment that was isolated from all cues to the time of day, eating and sleeping whenever they were inclined.

The researchers also collected skin cells from each person.

When they examined the DNA one variant stood out: a mutation in CRY1.

'Melatonin levels start to rise around nine or 10 at night in most people,' said senior author Dr Michael Young, head of Rockefeller's Laboratory of Genetics.

In DPSD patients, 'that doesn't happen until two or three in the morning'.

Researchers also discovered the mutation is dominant.

Currently, the best way for DPSD patients to control their sleep cycles is to stick to strict schedules or to get strong light exposure during the day.