What's your usual sleeping pattern? If you're happily tucked up in bed by 8 pm or even earlier, then you might have what's known as advanced sleep phase (ASP).
It's a sleep pattern that's traditionally been considered incredibly rare – after all, these so-called 'advanced sleepers' are naturally waking up at around 4 or 5 am, totally rested. But a new study of 2,422 people attending sleep clinics in the US has hypothesised that number might be much higher.
ASP leads to a shift in the body's circadian rhythm, which means the sleep-inducing hormone melatonin is produced by the brain much earlier in the evening than for other people. That's matched with a drop in body temperature.
The sleep study found that eight of the 2,422 participants they observed over nine years displayed ASP – that's around 0.33 percent, and far more than experts expected.
These results are limited to people who came to a sleep clinic for sleep issues, so they can't really be extrapolated to the general population. But based on those numbers, the researchers suggest that at least one in 300 people who attend a sleep study might be advanced sleepers.
While it's a limited sample, the results are interesting, because this is the first study to look into the prevalence of advanced sleep, and it also points to some genetic factors at play.
"While most people struggle with getting out of bed at 4 or 5 am, people with advanced sleep phase wake up naturally at this time, rested and ready to take on the day," says neurologist Louis Ptacek, from the University of California San Francisco (UCSF).
"These extreme early birds tend to function well in the daytime but may have trouble staying awake for social commitments in the evening."
The assessment was made after tracking visitors to a sleep clinic over a nine-year period. As well as records of sleep habits, the researchers also looked at the levels of melatonin in patients' saliva, plus more detailed polysomnography tests, which measure oxygen levels, heart rate and breathing during sleep.
Of the eight patients that the researchers identified as having ASP, five had close family members with the same sleeping pattern, suggesting that this is something that runs in the family – and may have a genetic basis. Further genetic testing found two of the relatives had genetic mutations linked to familial advanced sleep phase (FASP).
These advanced sleepers are more easily able to get up in the morning, and average a measly 5-10 minutes extra lie-in on non-work days, as opposed to the 30-38 minutes the rest of us average when we don't have to get up to start a shift.
One reason that ASP and FASP numbers might have been underestimated, the researchers suggest, is that people don't bother to report it or get treatment for it – they just live their lives waking up early and going to bed early.
In fact that 1 in 300 estimate might be low, as four other patients showed signs of having ASP but didn't want to take part in the study.
The downside of being an advanced sleeper is that they can tend to stay up long after they start getting tired in order to meet social or work commitments. That can lead to excessive tiredness the next day and a higher risk of associated health problems (like obesity and cardiovascular disease).
With balanced sleep habits so important for many different facets of our wellbeing, knowing exactly how many people have unique sleep patterns like ASP is going to improve assessments and treatments in the future.
"We hope the results of this study will not only raise awareness of advanced sleep phase and familial advanced sleep phase, but also help identify the circadian clock genes and any medical conditions that they may influence," says Ptacek.
The research has been published in Sleep.