Night owls who struggle to get up in the morning are often accused of falling into bad habits by their family, friends and workmates.
Now they have a new explanation for their reluctance to climb out of bed – they’re out of sync with the Earth.
Scientists have found that some night owls have a genetic mutation that means their sleep cycle lasts longer than 24 hours. As a result, those with the "night owl mutation" to the gene CRY1 stay up an average of two-and-a-half hours later.
It takes longer for their body temperature to fall so they can drift off at night, and for the body’s natural sedative, the hormone melatonin, to kick in. But night owls still have to get up for work at the same time as everyone else, leaving them drowsy and jetlagged.
US scientists say the gene discovery could lead to drugs to knock out the mutation, which could affect more than 85 000 people in the UK.
Lead author Alina Patke, from the Rockefeller University in New York, said: "Carriers of the mutation have longer days than the planet gives them, so they are essentially playing catch-up for their entire lives."
Principal investigator Dr Michael Young said: "It’s as if these people have perpetual jetlag, moving eastward every day. In the morning, they’re not ready for the next day to arrive."
While some may stay up late because they are disorganised, work late or simply because they are teenagers, the researchers found that up to one in 75 – among non-Finnish Europeans – have the gene mutation.
The discovery was made after they took different generations of an American family and shut them in a laboratory for two weeks.
These people had no way of telling the time, meaning they were forced to use their body clocks to decide when to eat and sleep.
A 46-year-old woman among them, found through a skin sample to have the mutation, had a circadian clock running half an hour longer than the normal 24 hours.
Melatonin, a hormone which rises when it gets dark to bring on sleep, normally does so at around 9pm or 10pm. But in the woman with the mutated gene, this did not happen until around 2 or 3am.
Someone’s core body temperature falls before they drift into deep sleep, which is why it can be easier to fall asleep in a cool room. But this too did not happen properly.
The researchers contacted other members of the patient’s family and discovered five relatives who shared the mutation. All of them had signs of delayed sleep, or a history of persistent sleep problems.